Support image justification

[cfreshman]

Allow for printing images (and bitmaps) with the current text justification setting

Usage:

printer.justify('C')
printer.printImage(img, justify=True)

It is especially useful for small images:

Notes:

• this follows the same pattern as the LaaT configuration, but could be implemented as a separate setting method as well
• printertest.py could showcase this option by centering the bottom QR code & Adafruit!, I can update this CR to include that if desired

[cfreshman]

Here's the potential update to printertest.py (not included in this CR yet): cfreshman@39713f7

[cfreshman]

There's a somewhat related issue for another image option, resizing: #6

That could be added as a keyword argument as well: cfreshman@7bf8b15

printer.printImage(img, resize=True)

I can create a pull request for that after this