Improve doing issue create with additional options

[timvink]

Looking at:

• glab issue create
• gh issue create

We could implement options like:

$ doing issue create --help
# ...
 -a, --assignee login   Assign people by their login. Use "@me" to self-assign.
 -b, --body string      Supply a body. 
 -l, --label name       Add labels (tag) by name. Space separate multiple labels.

Note we'll first have to implement:

• Renaming doing create issue to doing issue create (switch verb and context in commands #27).
• Implementing the @me user alias consistently everywhere (Implement @me as a user alias #36).