index most expandable attachments, fixes #331

Some things to ponder before merging:

1. If you have a hook for HTML decode, for example, your HTML attachments will get indexed. Which is great, except that it will probably slow indexing down a lot to make, for example, 100k calls to w3m or lynx.
1. Maybe someone would like to decode HTML but not have it turn up in searches they are doing for some reason.
1. Sup-sync doesn't notice when your email has gone from less indexable to more indexable, i.e. after you apply this patch or after you apply this patch and after you add an HTML decode hook. So, when you change something like that, for now, you'll need to delete your index and sup-sync again.

All that said, its pretty cool that you can add a decode hook and get indexing too. :)