Provide standard way to specify config file.

Added PreCommit::Checks::Plugin::ConfigFile to encapsulate standard
logic for specifying config file. Previously Rubocop used configuration
providers, JSHint used an environment variable or hardcoded file, and
coffeelint didn't allow a custom config file.

Now, the folling places will be checked for a config file.
* Config providers in priority order
* Plugin specified alternate location (Used by JSHint)

Adds the Env configuration provider, which will check
environment variables (JSHint manually did this previously).

Fixes #125