Add enhanced support for long running callback handlers. Add additional grpc options

[andymck]

This PR is linked to tsloughter/grpcbox#50

The changes originate from use cases encountered at helium and in integrating this lib into the helium stack. That said I believe the changes are general purpose enough to become part of the mainline branch

Change Summary

1. Updated service behaviours definitions, added {continue, grpcbox_stream:t()} response to streamed behaviours. Adds init/1 and handle_info/2 callbacks.

2. Support generation of PB files to use Maps or Props rather than enforcing Maps only

3. Add support for additional overrides:

proto_files: Allows a list of protos to be specified and used for PB and service behaviour generation, rather than defaulting to all protos in the directory specified by the "protos" option. If proto_files is not specified then defaults to using "protos"

create_services: boolean flag to determine whether or not to generate service behaviours.

beam_out_dir: allows a directory to be specified to where the beams including PBs and service behaviours will be outputted

keep_beams: the plugin builds the beams for the various proto modules and outputs to the 'ebin_dir'.
If this flag is set to true, these beams will be deleted after generation of the services. This is useful for example if the application wishes to generate erl files only and build all as part of the application build process

[andymck]

superseded by #14