Refactor key parser to handle crazy combinations.

Vim allows a mixture of <> notation and < or > characters without
problem, whereas Vrapper would force the user to write <LT> if a <
character was meant.

Implement a lot of back-out options to handle all cases, add test code.

Fixes #393.
Replace <LT> with < for simple cases.

Complex cases, e.g. those were you handle anything resembling a
specialkey still need to use <LT> and <GT>, but we no longer need it for
i< or '< testing.

Implemented to "dogfood" our fixes for #393.