Support for multi-digit numeric priorities

[JWorthe]

Hi there,

Firstly, thank you for your work on Orgize.

I came across an inconsistency between how Orgize and Emacs handle the priority cookie. Orgize currently only supports a single character priority. For example:

# Works, priority is A and headline is "High priority task"
* TODO [#A] High priority task

However, according to https://orgmode.org/worg/org-syntax.html section 4.1.1, the priority can also be a number between 1 and 64. Orgize supports single digit numeric priorities, but sadly priorities >= 10 fail:

# Works, priority is 1 and headline is "High priority task"
* TODO [#1] High priority task

# Fails, priority is None and headline is "[#42] Multi-digit priority task"
* TODO [#42] Multi-digit priority task

Cheers,
Justin