Correctly handles custom priorities

[nkakouros]

This solves #275.

It reads taskwarrior configuration and extracts the priorities from there (for instance, 0,L,,M,H, which treats L as lower priority that ''). The 'no priority' level is the empty priority (priority:). Priorities left of that are considered as insignificance indicators (and are shown with ¡, the reverse of !) and priorities right of the 'no-priority' are considered higher (and are shown as !, as is the case currently). For example:

0 -> Very Insignificant issue (¡¡)
L -> Insignificant issue (¡)
'' -> normal issue ()
M -> prioritize more (!)
H -> always come first (!!)

Of course, the above is just an example, the user can set the taskwarrior setting uda.priority.values to a sequence of characters so as to customize the exact behavior. In taskwarrior, it is the coefficient of a priority that matters and not the position of the the priority in uda.priority.values. This leaves the uda.priority.values settings free for the user to set in a way that the above markers (¡ and !) make sense to them.

I am also thinking of adding a configuration option to set custom markers (instead of ¡ and !).

One TODO is to highlight the (arbitrary) priority markers correctly, instead of only the !, !!, !!! cases.

[ghost]

Priority is UDA, which means one can even make it a float (like I have done).

I want to point out that a correct implementation of custom priorities should be included. As of writing, without this implementation, taskwiki fails silently.

[tbabej]

I'd argue urgency_levels is not a fortunate name here, as urgency already has a meaning in TW ecosystem which is distinct from priority.

[tbabej]

I'm not a big fan of the reversed exclamation mark - while it really does look cool, it is not straightforward to type on the keyboard. I'm wondering if there is not a different symbol we could use that would naturally convey the meaning of lower priority.