This CL adds support for extension in GYP.

This CL adds the extension target type and adds the link flags to
compile .appex extensions.

BUG=gyp:435
R=justincohen@chromium.org, mark@chromium.org, pkl@chromium.org, sdefresne@chromium.org

Review URL: https://codereview.chromium.org/376603002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1954 78cadc50-ecff-11dd-a971-7dbc132099af