Fix crash when starting without a config file

[mihaitodor]

• Add unit test to cover this scenario (TestNewConfigFromFlagSet)
• Refactor the code
  • let NewConfigFromFile return an error if the client name is not set. We don't care if addDefaultSubscription is called with an empty client name.
  • creating a dummyClient for each test which needs it is cleaner
  • goimports reflowed the files a bit

[mihaitodor]

Looks like my new unit test didn't work because CircleCI executes go test -v -race ./... and ExtractFlags kills the client if it finds a flag that it doesn't recognise (-v in this case).

I pushed another commit to refactor ExtractFlags to allow me to mock the command line parser.

[AlexisMontagne]

Sorry for the delay @mihaitodor, It looks good to me, i'll merge it now

[mihaitodor]

Thanks @AlexisMontagne! :)