Add --manifest option for build script

[jdecked]

Problem: The directory with our twa-manifest.json and the directory we're running the Bubblewrap commands from are different.

Solution: Add a --manifest option to the bubblewrap build command which allows the user to specify a directory where the client should look for twa-manifest.json, much like that option for the bubblewrap update command.

[andreban]

LGTM, Thank you very much for this!