Implement "custom" haml gettext parser

We currently use `GettextI18nRails::HamlParser` which is using `hamlit`
to convert `haml` to ruby and then `RubyGettextExtractor` (also part of
the `gettext_i18n_rails` gem) to extract externalized strings from the
converted ruby code. This however seems to be slower than the
`GetText::RubyParser` from the `gettext` gem. The performance difference
comes from `ruby_parser` vs `ripper`. The tool also seems to find more
strings, that the previous parser wasn't able to find.