Add defaults to VendorPrefixes config options in Readme

Change-Id: Ib182b9316987f448cc734b70fffcfed8e6e26258
Reviewed-on: http://gerrit.causes.com/44886
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Joe Lencioni <joe.lencioni@brigade.com>
Don't lint pseudo-selectors in SelectorFormat

The SelectorFormat linter shouldn't lint pseudo-selectors since these
must always use a hyphenated lowercase convention.

Change-Id: I1b3f65a8017d69c5f984b7721f626c9c27348e75
Reviewed-on: http://gerrit.causes.com/44887
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Fix Indentation handling of comments in @at-root

The linter would crash for code like the following:

    @at-root {
      // A comment
      ...
    }

This was due to the fact that comments don't have source ranges. Fix by
explicitly checking if `source_range` is not nil before attempting to
access it.

Change-Id: I8b1518458381689178b402fc8b9eb3c8dfa13100
Reviewed-on: http://gerrit.causes.com/44888
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Extract Options helper class from CLI class

The CLI class was becoming very unwieldy and hard to reason about.
Extract the command line options parsing functionality into a helper
class and change the signature of `CLI#run` to return the exit code
rather than have the method raise a `SystemExit` error itself.

Additional changes to come will extract more functionality from this
class to simplify it further.

Change-Id: I8432231ad6152313b085ef8de13bebbd22243918
Reviewed-on: http://gerrit.causes.com/44889
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>