Add check for linting HAML files via haml-lint

Add support for linting HAML files with `haml-lint`. This leverages the
power of Rubocop for linting the Ruby found in HAML files by extracting
the Ruby code and running it against Rubocop.

Like the RubyStyle check, this uses whatever `.rubocop.yml` file the
`rubocop` executable finds first, allowing users complete control over
which lints get reported.

Change-Id: Ie48aabdf8d05a2df4b2bf66e04d7ff973ab7cb66
Reviewed-on: https://gerrit.causes.com/28525
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>