Experimental conversion control

[richfitz]

This PR adds an experimental and subject-to-change interface to the low level redis commands to control how string conversion happens, to address #60. You can run any redis command by using the lower level interface:

con <- hiredis()
con$command(list("PING"))
#> [Redis: PONG]

So we can already set data this way:

con$command(list("SET", "key", "value")) # same as con$SET("key", "value"))
#> [Redis: OK]

and get data this way:

con$command(list("GET", "key")) # same as con$GET("key")
#> [1] "value"

The new feature added in this PR is a 3rd argument to $command, as, which is a string taking values auto (the current behaviour) or raw (the default NULL is equivalent to auto). In action:

con$command(list("GET", "key"), "raw")
#> [1] 76 61 6c 75 65