Add a Drush command for changing an external library URL

Often it may be necessary to change the URL of an external JavaScript library (for example, if a new version of the library was released and you want to switch your site to use it). It would be useful to be able to do that via Drush, to facilitate e.g. upgrading a bunch of sites at once in a multisite installation.

The attached patch implements a Drush command with this functionality. Note that #1370790: Allow external libraries with a .txt extension must be committed first in order for this patch to work.