Add a defcustom for `magit-status-mode-hook'.

This will allow users to customize it and extension packages, such as
magit-filenotify, to use `custom-add-option' to add themselves as option
to it.
Add `magit-load-config-extensions' option to `magit-mode-hook'.

This will make it easier and more noticeable when customizing magit.
Add options for stgit, svn, topgit to `magit-mode-hook'.

This will make them more noticeable when customizing magit and easier to
enable.
Merge pull request #1456 from ruediger/customhookoptions

Provide options for `magit-mode-hook`