Support external branches like svn:externals

Using hooks, this plugin supports external branches for these commands:

* branch
* checkout
* commit
* pull
* push
* update.

Works from any client including bzr, QBzr, TortoiseBzr and Bazaar Explorer.
To begin work with external branches please see ``bzr help externals-add``.

This plugin also provides a command to execute an arbitrary bzr or shell
command for each branch in the local workspace, see also
``bzr help externals-command``.

Works with snapshot external branches only for ``branch/checkout`` command with
revision options. The snapshot becomes at commit in the main project.

Known issues:

* Does not support ``^/`` expression in relative urls.
* pull command does not overwrite your newer working tree if you use a custom
revision.
* update command does not support a custom revision and does not check
externals if the branch tip is not changed.
* Does not support lightweight checkout.
* qcommit command does not support external snapshots via hooks, instead of it
use ``ecmd qcommit`` or simple ``commit`` commands.

Plugin used config files::

.bzrmeta/externals
.bzrmeta/externals-snapshot

Format of config files the following::

url directory [revisionspec]