[vcsh] hook system for vcsh

[Vincent Demeester]

Simple and efficient (at least better than my hook-support branch).
In my opinion it would just need more hook (in init and/or clone).

> i would suggest to add a file extension like .sh too, so that syntax hilighting works in editors

This could be a tip but I don't see the need to add an extension. With decent
editors, syntax highlighting can be set with the header comment (#/bin/bash) or
with editor mode-line (#vim: syntax=zsh, or # -*- mode: sh *-*, …). Anyway the
hook function doesn't need to limit to an extension 'cause it could be written
in any language.

Vincent