[rss] Add a docstring to main command function

This somehow got overlooked when refactoring.

Also added a doc line for fg/bg colours.
[rss] Avoid error looking for HTTP status when fetching non-web feed

Feedparser will not return a status attribute if fetching from a feed on
disk or in memory, so added a check for its existence.
Merge pull request #335 from saltire/rss

[rss] Improved documentation, fix error when parsing non-web feed