Fix case typos in example bib
Make all citekey searches case-sensitive

While this was implied in the docstring of `bog-citekey-format', it
wasn't actually true.
Remove leading digits from bog-citekey-format

I originally allowed leading digits to account for a case where a
consortium's name started with a number.  However, looking back at this,
I don't think it makes sense to change the default format for such a
rare case.
Modify bog-citekey-format docstring
Anchor and restrict citekey regular expression

Limit the characters that are allowed in the citekey to letters, digits,
underscores, and hyphens.  This allows the citekey to be treated as a
word (temporarily expanding word constituents to include underscore and
hyphen), as opposed to explicitly specifying the characters that can
come before a citekey.
Clarify bog-citekey-file-name-separators docstring
Fix specification of defcustom variables

- Fix a few places where the group or type was missing.
- Use 'directory' or 'file' instead of 'string' when appropriate.
Add commands bog-{next,previous}-non-heading-citekey
Make citekey groups for web search customizable