Allow configuration from package.json, in line with JSHint.

JSHint now allows configuration directly from `package.json`. This pull request simulates this behaviour.  If a config file is given as a command line option, then an attempt is made to load it and nothing else. If not, then attempts to load config are made in this order:

 - `package.json` jscsConfig field.
 - `.jscsrc` configuration file.
 - `.jscs.json` configuration file.