Feature Request: allow an option or selection list

[sigzero]

It would be nice to be to able to present multiple items to select from when the snippet section is called.

For example, for a shell script, I would define a TODO list like:

# ${1|TODO,FIXME,BUG,XXX|} - ${2:synopsis}

I've taken the construct from how vscode does it. So using the pipe character to enclose a comma delineated list gives a popup list to make a selection from.

[ajzafar]

This is already implemented in the completion branch. There are just a couple bugs that need working out before it's merged to master.