Added docs for Vebugger_PreUserAction and Vebugger_PostUserAction

[TamaMcGlinn]

In issue #9 you mentioned adding custom command hooks for users to subscribe to, but I couldn't find anything in the documentation for it.

[TamaMcGlinn]

By the way, I went into this because I wanted to see if I could add a seperate window that keeps track of the stack, allowing you to move up and down it and navigate with enter, similar to how NetRW works with a buffer in a non-text mode.

It would read the args and see if the command is one of a given list that is known to change the stack, and if so, call backtrace again to refresh that other buffer.

Now that I've read the documentation, I am wondering whether adding a think handler might be the better approach. Any thoughts on that? In fact, are there any cases where subscribing to Pre or PostUserAction is preferable to adding a think handler?