syntax-erb: Make injections more lenient

This allows ERB tags to be opened inside of HTML tags.
syntax-ruby: Allow underscores as valid string delimiters

The underscore character can be using as a surround delimiter in any `%{` enclosed strings.

E.g.

    foobar = %q_this is a #{valid} interpolated string_

The above string is valid Ruby syntax.
Merge remote-tracking branch 'upstream/master'