Adafruit GFX updates

[sticilface]

Any chance you could update this lib to the newer adafruit GFXlib.

It supports things like returning the size box that a given string will fill. which would be very useful.

many thanks

[Adrianotiger]

getTextBounds is really important if you want center the text in the screen.

[ImpulseAdventure]

For reference, the merge of PR #54 might address some of the intent behind the requests above.