Added support for matching using context managers.

[aarondewindt]

Proposed implementation for the context manager based pattern matching described in issue #37

[aarondewindt]

I think its only failing with the type checks. I'm not sure I broke something since the same errors appear to be part of existing issues. I'm not sure though.

[jspahrsummers]

Thanks for the idea and the pull request! I think this is pretty compelling.

I'm fairly short on time these days, so I'm afraid I can't help much in implementation, but I do believe all the tests pass before these changes (any tests expected to fail are marked as such and don't block CI). You may need to update the mypy plugin to support this new idiom.

Would you be willing to make those changes, so that it's all green? Then I'm more than happy to merge this! 😄

[aarondewindt]

I'll have a look at it in the coming days. I have no experience developing mypy plugins, so it might take a while.