Integrate YCMD JSON settings

Add `ycmd-settings-json-filepath` - path to YCMD settings JSON file.
YCMD accepts a json file where the server settings are set, including
a one-time HMAC. The file is deleted after the server starts.
As for now, emacs-ycmd was just creating a temporary file from
some hard-coded/customizable values.
Since those settings have to direct relation to emacs-ycmd, rather
only to YCMD, it is good idea to keep it this way and just feed the
file adding an HMAC value.

That change will read the JSON file, insert HMAC and pass it to YCMD.
Please, test it, feedback is appreciated, I will mainly test it on
C/C++ and Go.

That is supposed to solve #472, #479.

ycmd.el

@@ -130,6 +130,10 @@
   "Path to global extra conf file."
   :type '(string))
 
+(defcustom ycmd-settings-json-filepath nil
+  "Path to YCMD settings JSON file."
+  :type '(string))
+
 (defcustom ycmd-extra-conf-whitelist nil
   "List of glob expressions which match extra configs.
 Whitelisted configs are loaded without confirmation."
@@ -2077,61 +2081,18 @@ This produces output for empty alists that ycmd expects."
 (define-hmac-function ycmd--hmac-function
   ycmd--secure-hash 64 64)
 
-(defun ycmd--options-contents (hmac-secret)
-  "Return a struct with ycmd options and the HMAC-SECRET applied.
-The struct can be json encoded into a file to create a ycmd
-options file.
-
-When we start a new ycmd server, it needs an options file.  It
-reads this file and then deletes it since it contains a secret
-key.  So we need to generate a new options file for each ycmd
-instance.  This function effectively produces the contents of that
-file."
-  (let ((hmac-secret (base64-encode-string hmac-secret))
-        (global-config (or ycmd-global-config ""))
-        (extra-conf-whitelist (or ycmd-extra-conf-whitelist []))
-        (confirm-extra-conf (if (eq ycmd-extra-conf-handler 'load) 0 1))
-        (gocode-binary-path (or ycmd-gocode-binary-path ""))
-        (godef-binary-path (or ycmd-godef-binary-path ""))
-        (rust-src-path (or ycmd-rust-src-path ""))
-        (swift-src-path (or ycmd-swift-src-path ""))
-        (racerd-binary-path (or ycmd-racerd-binary-path ""))
-        (python-binary-path (or ycmd-python-binary-path ""))
-        (auto-trigger (if ycmd-auto-trigger-semantic-completion 1 0)))
-    `((filepath_completion_use_working_dir . 0)
-      (auto_trigger . ,auto-trigger)
-      (min_num_of_chars_for_completion . ,ycmd-min-num-chars-for-completion)
-      (min_num_identifier_candidate_chars . 0)
-      (semantic_triggers . ())
-      (filetype_specific_completion_to_disable (gitcommit . 1))
-      (collect_identifiers_from_comments_and_strings . 0)
-      (max_num_identifier_candidates . ,ycmd-max-num-identifier-candidates)
-      (extra_conf_globlist . ,extra-conf-whitelist)
-      (global_ycm_extra_conf . ,global-config)
-      (confirm_extra_conf . ,confirm-extra-conf)
-      (max_diagnostics_to_display . 30)
-      (auto_start_csharp_server . 1)
-      (auto_stop_csharp_server . 1)
-      (use_ultisnips_completer . 1)
-      (csharp_server_port . 0)
-      (hmac_secret . ,hmac-secret)
-      (server_keep_logfiles . 1)
-      (gocode_binary_path . ,gocode-binary-path)
-      (godef_binary_path . ,godef-binary-path)
-      (rust_src_path . ,rust-src-path)
-      (swift_src_path . ,swift-src-path)
-      (racerd_binary_path . ,racerd-binary-path)
-      (python_binary_path . ,python-binary-path))))
-
 (defun ycmd--create-options-file (hmac-secret)
   "Create a new options file for a ycmd server with HMAC-SECRET.
 
 This creates a new tempfile and fills it with options.  Returns
 the name of the newly created file."
   (let ((options-file (make-temp-file "ycmd-options"))
-        (options (ycmd--options-contents hmac-secret)))
-    (with-temp-file options-file
-      (insert (ycmd--json-encode options)))
+        (json (json-read-file ycmd-settings-json-filepath))
+        (hmac (base64-encode-string hmac-secret)))
+       (assq-delete-all 'hmac_secret json)
+       (setq json (json-add-to-object json "hmac_secret" hmac))
+       (with-temp-file options-file
+         (insert (ycmd--json-encode json)))
     options-file))
 
 (defun ycmd--exit-code-as-string (code)