Add 'rulesFile' option to config properties

[tingley]

First attempt at a fix for issue #280.

This adds a new option, rulesFile, to the server configuration file specified with --config. The rulesFile option points to the location of a .languagetool.cfg-style rules file.

This is a little odd (the config file now points to another config file), but my thinking was that it was better to keep the server options consolidated in the --config file, rather than adding a new top-level switch. I am also allowing the rulesFile param to point to an arbitrary file location, since that's more convenient for server deployment scenarios where we don't want to depend on things like the location of the home directory of the "user" running the service.

I'm happy to rework any portion of this if you'd like it to work a different way.

[danielnaber]

Thanks, looks good. I'll merge in a few days once we're out of feature freeze for LT 3.0.