Allow escaping values defined in "docstring statements" such as tags

[clebergnu]

Time and time again we expand the definition of the DOCSTRING_DIRECTIVE_RE_RAW in an attempt to allow for more descriptive tags.

This time there's an argument in favor of allowing commas to be used as part of tag values.

To cover this and future use cases, let's evaluate (and if possible) come up with a general and universal way of escaping characters in the docstring statements.

[wainersm]

It is hard to keep backward compatibility.

Possible solutions:

1. Introduce a "tag_separator" tag where the user can specify the separator character (e.g the -d option of cut).
   Advantages: suppose only a few tests need a non-commas separator, so you have fine-grained control
   Disadvantages: the other way around, all tests need this new separator, so you end-up with "tag_separator" everywhere
2. Allow the user to define the separator on any avocado.conf file. It would need to allow a avocado.conf at the project-level because, IIRC, currently that file can live at system or user's home only. The advantages/disadvantages are the inverse of 1), so maybe the two solutions could coexist.