Make it possible to create custom hint commands

Make it possible to create custom hint commands

My first idea was to use `vim.enterMode('hints', ...)`. However, that's
too complicated to be useful for any user.

Actually, I found that `vim.enterMode(...)` is too complicated for most
(all?) modes, so I decided to make it private (removing it from the
public API) while at it. (I also made the arguments of
`vim.enterMode`/`mode.onEnter` more consistent with each other.)

I also considered exposing the entire "API" that the `f` commands use
internally. However, that's more complicated than the average user
needs, and would increase the API surface _a lot._

Instead, the `f` commands now take a `callbackOverride` argument. This
lets you choose the `f` command that matches the elements you're looking
for and the override what happens when a hint marker is matched.

Fixes #785. Fixes #654.