[Feature] Add a help script for packaging external extensions

Create extension_manager.py to help developer pakcage external
extensions.

Usage:
python extension_manager.py \
--add=https://github.com/shawngao5/crosswalk-iap-extension.git
python extension_manager.py --remove=iap
python extension_manager.py --list
python extension_manager.py --search=ia*

BUG=XWALK-1947