Rule-based preferences - format marker RFC, etc.

[Martin Pool]

I spoke with Ian a bit about that this morning.

We plan to put a namespace in the section headers something like

 [name *.py]

('name' vs 'glob' vs 'pattern' etc Ian is going to ponder)

The 'require' thing seems like a generally useful mechanism for people
where correct operation depends on having some plugins or similar
things active.

I don't think this needs special handling by file id.  In general we
need to do many lookups by filename and we should try to make that
fast.  Let's land this.

-- 
Martin <http://launchpad.net/~mbp/>