Improve script to build featured article collections.

Change-Id: I2969662ecd0fa7de5a08c910efac8935803cdc02
Further improvements and bugfixes to featured article bundling.

Trim pathnames (where necessary) intelligently; don't chop off extension.

Allow specification of metabook collection title on command line; use this
in the samples/featured.js collection generator.

Remove _ from article titles when generating metabook.json.

Change-Id: I88c7d39923976e0053b822784c40033ec8fd8f84