Allow unnamed capture groups in regex and urlg

[ComaVN]

Allow unnamed capture groups to be used in the regex and replacement url. All regexes used by trac and plugins for wiki-formatting are put into one big regex, meaning the numbering of unnamed groups is unpredictable.

It should be possible to use unnamed capture groups in the regex:

\bexample(\d+)\b

that can then be used for replacement in the url:

http://example.com/\1

or, when the replacement is followed by a digit:

http://example.com/\g<1>0

[ComaVN]

(In [4867]) added test for unnamed groups (seems to work already?) closes #4146