Add support for translation context in HAML

[adam-h]

Resolving: https://github.com/grosser/gettext_i18n_rails/issues/199

Note that this removes support for very old versions of gettext too as this uses the ruby parser which was refactored in 3.0.0 (2013). The development dependency is 3.0.2 so I think this should be OK.

See individual commits for details, key change is in:

Switch out haml and slim parsers to use GetText::RubyParser

We now just parse the haml or slim to ruby code and then hand it off to the GetText::RubyParser to actually extract the strings into the PO format.

This adds support for translations with context (p_("My Context", "id_key")) and removes the custom ruby extractor entirely.

It also fixes the 'pending' spec for slim to parse ruby 1.9 syntax.

[grosser]

nice!

[grosser]

1.12.0 🤞