Add poetry plugin

[nat-n]

Based off of #32 by @ThatXliner and refactored a bit.

• Uses the new command structure support in Poetry 1.2 to register commands with poe as a prefix. Also gives users the option to remove this (in which case they need to be careful not to have tasks that collide with poetry commands), or set it something else like "run " or 'x " etc.
• Uses the virtualenv from poetry directly rather than calling poetry again in the task subprocess.
• Uses the default verbosity level from poetry unless it's set in pyproject.toml
• Includes a best effort hack to make open ended CLI options work with cleo via monkey patching
• Includes some enhancements for testing code

[nat-n]

There's still an open question about how to handle passing of arguments to tasks. This doesn't work out of the box since tasks are registered as commands using cleo as a CLI library. I'm afraid that it may be necessary to refactor poethepoet to use Cleo throughout! Though this might still not help for passing arbitrary args to cmd tasks for instance. I haven't looked into this properly yet.

[ThatXliner]

Ooh this looks promising. So you have the option to make it a prefix now?

Also, I'm a bit confused on the poetry_prefix configuration... What does it do? What does it mean?

[ThatXliner]

Looking good! Haven't tested it on my machine yet.

Looking from the code, I guess you got the plugin to invoke the PoeThePoet object.

[nat-n]

@ThatXliner I think it's almost ready to go. If you could test it out a bit then that would be helpful :)