Allow text to be marked exempt from spell-checking

[rrthomas]

Currently, each file has its own local words list. Instead, use a project-wide dictionary.

Also, find some way to mark one-offs "sic" (e.g. "Eeehhhh", foreign words) so that they don't need to be even in the per-file list.

[rrthomas]

A project-wide wordlist has been added.

Can use the technique here: https://stackoverflow.com/questions/8287330/exempt-code-chunks-in-an-sweave-document-from-emacs-spell-check to add "sic" markings for exclamations, foreign words etc. Since in LaTeX it will require extra line breaks to make suitable comment markers, it's probably only useful for whole phrases/sentences.