Remove Linter superclass from LinterRegistry

The superclass `Linter` was incorrectly being returned in the list of
registered linters. Remove the inclusion of the `LinterRegistry` module
to remove it.

This didn't previously have any negative effect because the `Linter`
class `run` method is effectively a no-op.

Change-Id: I1199f0b35ff16f693d3d17e99288cd93add4200b
Reviewed-on: https://gerrit.causes.com/27622
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Kim Douglas Clarence Mason <kim.mason@causes.com>
Add show-linters flag

Add `--show-linters` flag which prints out the list of all linters
registered with `scss-lint`.

Change-Id: I909d15f1e8bfc631d64af3822717833a7d6548c4
Reviewed-on: https://gerrit.causes.com/27623
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Kim Douglas Clarence Mason <kim.mason@causes.com>