Control over ENUM values

This version introduces a new converter flag / option which controls the types of values generated for ENUMs. The default behaviour is still to generate strings AND numbers, but the new flag / option limits this to the string values only.

See the README for details and examples:

• https://github.com/chrusty/protoc-gen-jsonschema#configuration-parameters
• https://github.com/chrusty/protoc-gen-jsonschema#enum-options

Thanks @mrozycki-tink for your contribution!