Allow recursion into {pre,post}-up hook directories

When I found the hooks, I found that `rcm` only checks for an executable file (`pre-up` or `post-up`). Following some common sense I imagined that executable files inside those directories (something like the git hooks) should work too.

It's quite precarious, and variables could be a bit better, but works fine.

If you like it, I can add it this feature to the documentation and make the pull after that.