Add tests

[Soares]

Tests would be nice.

Vimdoc is currently pretty hacky and fairly buggy.

[dbarnett]

Vimdoc also has a side-effecty architecture that makes several of its existing limitations hard to solve. Separating the logic out into a parsing phase that builds a traversable model of the detected code and a generation phase that has access to that full model would enable us to knock out a bunch of these issues.

[dbarnett]

And I guess independent of refactoring, we'll want integration tests that say "given this example plugin, vimdoc should produce output containing X, Y, and Z".