Add 'autocheck' and 'type' support to menuitems

I needed `autocheck` support in my project, which in turn needed `type` to be set correctly, so here is a patch adding them, with some drive-by fixes to typos in comments/variables.

I couldn't get tests to run properly, this might be an issue with the version of addon-sdk I'm using. I want to add more, but I figured I'd submit the PR to get an idea if this is something you'd want to have in the project first.