#7 various issues with promptsys

[Adam Spiers]

The following need to be dealt with:

- allow support for theme configuration via zstyle
- in theme configurations, allow choice between bold/nobold colours
- make sure no theme needs a particular font by default
(i.e. avoid 8 bit characters by default)
- allow `gray' to mean the same as `grey'
- stop themes stomping on precmd
- tidy up prompt_opts mess

[Bart Schaefer]

Also: Better handling of precmd/preexec. Consider example in prompt_bart_setup.