Also ensures we check Syntax with RuboCop, which is different.

Files in lib will be eager loaded and hence will require haml_lint to be
loaded. Since this is only a development dependency, we can't assume
this gem will be available in production, so it should never be loaded
in production.

Add a simple haml_lint linter to report all plain nodes.
"Plain nodes" in HAML are scritpless plaintext leaf nodes.

Add haml-lint_todo to onboard new haml linters.
Excludes existing NoPlainNodes lints.

When using ruby interpolation in externalized strings, they can't be
detected. Which means they will never be presented to be translated.

To mix variables into translations we need to use `sprintf`
instead.

Instead of:

    _("Hello #{subject}")

Use:

    _("Hello %{subject}) % { subject: 'world' }

Signed-off-by: Rémy Coutable <remy@rymai.me>

Signed-off-by: Rémy Coutable <remy@rymai.me>