Write documentation

We are adding every time more features (cool!), but we do not really document them (uncool). We should. Do we want to use github's wiki for it? We should also write a man page, in spite of its ugly syntax.

(Probably incomplete) List of things to document:

• [ ] FTS installation
• [x] Full list of options with explanation of options
• [x] FTS syntax, including the columns that can be searched in bibsearch
• [x] Config file options
• [x] 'custom_key_format' format
• [ ] FAQ with at least "I installed sqlite3 with FTS but it is still complaining" and "my searches that include '-' or ':' fail miserably"
• [ ] Call for help for .bib files

A man page might be good. I like the approach of writing an extensive README and converting that to a man page somehow, or having bibsearch help pull it in somehow. I'll take a stab at this.