@task-controlled shortflags/flag aliases

[bitprophet]

Argument-level API allows for an arg to have one or more names, but right now @task only does auto-shortflags or no-auto-shortflags without exposing the aliasing behavior to users.

Obvious choice is a map, @task(argnames={'realkwarg': ['r', 'rk']}) (turning into the parser-level flags for realkwarg being --realkwarg, --rk and -r). Not sure what this option's name should be though, argnames is kinda fugly.

[bitprophet]

Rolling into #174