[Feature Request] Add "digits only" symbol set

[trolley813]

For 12x12 (and potentially larger sizes such as 16x16 etc), it would be useful to have an option to display numbers instead of single character variations (i.e. 10, 11, 12 instead of A, B, C etc.).

The changes seem to be trivial, as seen from the code. I can make a PR for this some time later.