Consolidate the globals into a struct that is available via a `Globals()` function, nudge
us towards a slightly more sane direction for handling global config. Adds the ability
to set decoder pool size and plugin input channel sizes on the command line.
Update docs to reflect new command line flags.
Merge pull request #101 from mozilla-services/features/globals-struct

Slightly better handling of some global config

Resolves issue #84.