go/doc: Detect headings in comments and format them as h3 in html.

To structure larger sections of comments in html output headings
are detected in comments and formated as h3 in the generated html.
A simple heuristic is used to detect headings in comments:
A heading is a non-blank, non-indented line preceded by a blank
line. It is followed by a blank and a non-blank, non-indented line.
A heading must start with an uppercase letter and end with a letter,
digit or a colon.  A heading may not contain punctuation characters.

R=jan.mercl, gri, adg, rsc, r
5 files changed