Move helper methods out of lib/scss_lint.rb

Investigation into other gems reveals that the root file in lib
typically only loads other code needed by the gem.

With this in mind, remove the code defined here into `CLI` (since it was
only used by that class anyway) and convert `autoload` statements into
`require`s so that we don't need the containing `module` declaration.

Change-Id: I932856758b76530c0f9a2f5979388f1151f0f63f
Reviewed-on: https://gerrit.causes.com/30811
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Hao Su <hao.su@causes.com>