Add commandline flag to configure what section of the documentation to use

[gerritholl]

Currently, viirs_dr_runner.py is hardcoded to look for a section [dev] in the configuration file passed on the commandline. This is undocumented and not user friendly. I propose to add a commandline option -C, similar to the one used by trollstalker in pytroll-collectors, that instructs what section from the configuration to use.

I am open to other suggestions on how to improve the status quo.