Add backlight inversion option (Fixes #5)

[sajattack]

I saw this issue going through @kattni's library status issue and it looked like a quick fix so I jumped in. The new API lets you add an inverted field to the backlight object used in the constructor, and also lets you specify in the set_backlight method. In both cases it defaults to True, the old behaviour, so this should be a non-breaking API change.

[tannewt]

Thank you for the PR! I wouldn't add the attribute to the backlight object because its type is unknown. Instead, I'd have it separate state that is added to self via the constructor-only. It doesn't need to be in set_backlight because it won't change after it's hooked up.

[brennen]

I think @kattni may have a slightly different fix for this issue baked into some work she's doing on this library, which is getting a pretty substantial overhaul for unrelated reasons. Probably should wait for her to comment on this one.

[kattni]

@sajattack I am currently doing a complete rework of this library. It will include the fix for the backlight issue. I really appreciate you putting in this PR, but I'm going to be taking care of the fix myself as part of the rework. Thank you again for your work!