FEATURE: added strip_sdk option to mozilla-cfx-xpi tasks

This change adds an explicit strip_sdk option to the "mozilla-cfx-xpi" grunt multi-task 
(as a follow-up of the suggestion from #11).

It is true by default, which means that by default the add-on xpi is smaller and it does not contain a private copy of Mozilla Addon SDK module sources, but the addon will use Addon SDK modules already integrated in the Firefox sources.

It should be set to false, if our add-on is based on a custom patched version of Mozilla Addon SDK or if we're using a module from an Addon SDK version which is not already integrated into the targeted Firefox version.