Add a simple blacklist

[davidshepherd7]

Fixes #29

[Trevoke]

This is fantastic. The last bit is to write a little documentation for this (I usually just write it once and duplicate the text in the README and in the top comment block of sqlup-mode.el.

The documentation can be very sparse: we're in pre-1.0, still, and this is literally an experiment to see if it's good enough as a feature for package users.

Are you up for it? If not, I can do it.

[davidshepherd7]

Oops yeah, I was intending to add some documentation but then forgot, done now.

[Trevoke]

Awesome, thank you!