Add verbose mode

[dsebastien]

It should be possible to:

1. pass a --verbose (or -v) argument when executing gulp and have that trigger additional logs (e.g., files in streams, etc)
2. configuration an option to enable debug/verbose mode (cfr Add configuration hooks #8)

[dsebastien]

The args could be retrieved at the entry point using the yargs module (https://www.npmjs.com/package/yargs) and passed within the options object (e.g., options.args)