Include selection keyword details in user guide

Include selection keyword details in user guide

- Make 'gmx help -export rst' write out the detailed selection keyword
  help as subsections of the keywords topic.
- Separate the titles from the help texts to make it possible to format
  the title separately.
- Make synopsis formatting the same for each keyword details help topic.
- Make HelpWriterContext not write leading empty lines to console output
  when the help text starts with a literal block.
- Remove unnecessary stars from the keyword lists in the
  reStructuredText output.

Some minor cleanup of the formatting could be done, but otherwise this
closes #679.

Change-Id: Id91ce3c01e9dd5f1c068498217dbc536a712c231