help make better topics

[pchampin]

a common practice in some groups is to use issue/PR URLs as (sub)topics.

Currently, this cause GHURLBot to utter the title of the issue or PR, which is useful.
But in the TOC, the (sub)topic still shows as a URL (possibly shortened to owner/repo#123), which is not very nice.

To improve that, it would be nice if GHURLBot behaved slightly differently when reacting to a topic or subtopic, and utter a substitution command instead.

More precisely, this is what happens currently:

16:22:53 <pchampin> subtopic: w3c/rdf-tests#135
16:22:54 <gb> https://github.com/w3c/rdf-tests/pull/135 -> Pull Request 135 Updates Turtle tests to use triple term syntax and reifications (by gkellogg) [enhancement] [Turtle]

With the proposed change, this is what would happen:

16:22:53 <pchampin> subtopic: w3c/rdf-tests#135
16:22:54 <gb> s|subtopic: w3c/rdf-tests#135|subtopic: https://github.com/w3c/rdf-tests/pull/135 -> Pull Request 135 Updates Turtle tests to use triple term syntax and reifications (by gkellogg) [enhancement] [Turtle]

Ideally, this would also happen when people use the full URL of the issue:

16:22:53 <pchampin> subtopic: https://github.com/w3c/rdf-tests/pull/135
16:22:54 <gb> s|subtopic: https://github.com/w3c/rdf-tests/pull/135|subtopic: https://github.com/w3c/rdf-tests/pull/135 -> Pull Request 135 Updates Turtle tests to use triple term syntax and reifications (by gkellogg) [enhancement] [Turtle]

To be clear: substitution commands would only be used when somebody says "topic: " or "subtopic: " followed by a github issue (as recognized by GHURLBot or as a full URI) and nothing else. I don't expect GHURLBot to handle more complex cases...