patman: Support parsing of review snippets

Add support for parsing the contents of a patchwork 'patch' web page
containing comments received from reviewers. This allows patman to show
these comments in a simple 'snippets' format.

A snippet is some quoted code plus some unquoted comments below it. Each
review is from a unique person/email and can produce multiple snippets,
one for each part of the code that attracts a comment.

Show the file and line-number info at the top of each snippet if
available.

Signed-off-by: Simon Glass <sjg@chromium.org>