Specify configBridge as relative path to source

Specify configBridge as relative path to source

This adjusts the load path for the ``configBridge.json`` file from one
that's relative to the user running the command (eg ``jekyll serve``),
to one that's relative to the ``source`` configuration setting for
Jekyll.

The result is that the user can now have a (customised) ``_config.yml``
for Jekyll anywhere on her filesystem and point to Bootstrap's
``docs`` directory to use as the ``source``.  Previously, in order to
customise it, the original ``_config.yml`` needed to be modified inside
(a forked) Bootstrap, and the ``jekyll`` command could only be run at
the root of the Bootstrap package as the original file path to
``configBridge.json`` was only valid there.

The existing behaviour is not affected.