Add operator for REGEX matching in context about fmf HOT 1 OPEN

Let me add to this, the regex matches should be exposed as context objects:

• by default all regex matches should be stored under a regex_match context
• for clarity, the regex should only be allowed to match once so regex_match is singular valued
• regex match groups are stored as the dictionary of regex_match
  • always save the matches by their group int keys
  • if the group is named, include the str key-values
  • there is always the match group 0 that is the full regex match

This could be particularly useful in combination with #227

from fmf.