Fully support extensions that add GLuaTest "flavors"

One benefit of having GLuaTest operate on tables is that we don't really care how the end user actually _makes_ the table.

With this in mind, we could actually create "flavors" of GLuaTest syntax that more closely match the syntax that people have come to love in other frameworks.

Some ideas:
- [Jest](https://jestjs.io/)
- [RSpec](https://rspec.info/)
- [xunit](https://github.com/xunit/xunit)

To accomplish this, GLuaTest will need to have a sufficient number of hooks that allow Extensions to modify anything they'd like. Off the top of my head this means, certainly:
- Some way to add globals to test files when they load.. somehow
- A handler hook that passes the raw table received when loading a test file

Best way to actually figure the rest of these out is to just try making one of these flavors. I'll probably start with Jest or RSpect.