Include minitest-stub_any_instance under known extensions

The [pull request to add this to minitest](https://github.com/seattlerb/minitest/pull/245) was denied, but @zenspider said it would be suitable as a gem. @nashby declined, but I felt it was worthy enough to package the code for others' easy reuse.

[gem on rubygems.org](http://rubygems.org/gems/minitest-stub_any_instance)
[code on github](https://github.com/codeodor/minitest-stub_any_instance)