config: add gitosis.extProps = <rel_to_home>/<ext>.py

The <ext>/py must contain a get_props functions which is supposed to
return a tuple of subclasses of RepoProp.

More words about these subclasses of RepoProp:
* Override the _get(self, config, reponame) method:
  a) Returns the value of this property.
  b) If returns None, the action method won't be invoked.
* Override the action(self, repobase, name, reponame, val)
  a) @repobase is the repositories dir.
  b) @name repositories relative path with '.git' stripped
  c) @reponame name of the repo which @name belongs to
  d) @val is returned by _get()