issues: support generic regex replacements in issue_url and issue_prefix

Issue reference linking is pretty limited:
- the issue_url is a literal with only three special tokens {id},
{repo} and {repo_name}. There is no way to let the URL be dependent on
other elements of the input issue reference.
- The value for {id} is somewhat oddly determined by the concatenation of
all parenthesized groups in the issue_pat regular expression
- the link text of the resulting link is limited to the contents of the
literal issue_prefix with the determined {id}. It is not possible to
retain the input issue reference verbatim, nor to let the link text be
dependent on other elements of the input issue reference.

This commit makes the issue reference linking more flexible:

- issue_prefix is replaced by the more generic issue_sub(stitution), which
is a string that may contain backreferences to regex groups specified in
issue_pat. This string, with backreferences resolved, is used as the
link text of urlified issue references.
- if issue_sub is empty, the entire text matched by issue_pat is used as
the link text.
- like issue_sub, also issue_url can contain backreferences to regex groups.
- {id} is no longer treated as a special token, as it can be solved by
generic backreferences ('\g<id>' assuming issue pattern contains something
like '(P<id>\d+)'. {repo} and {repo_name} are still supported, because
their value is provided externally and not normally part of the
issue pattern.

Documentation and ini file template is updated as well.