Bug 1084006 - Allow hosts to specify their own CLI arguments

This is the first step in a pretty major overhaul of how hosts work this does away with all the terrible hacks we added so far to do exactly this. This also aims at removing the need for ./bin/gaia-marionette. The manifest functionality has been moved into its own binary (and can be used via some shell subs).
In part it is to workaround some issues (i.e. me being lazy) separating out options from files in argparse but this is also the first step into removing the manifest parsing from marionette-js-runner... I think the logic belongs elsewhere or we should use the same format elsewhere at mozilla.