Allow to set everything both with a command line parameter or an environnement variable

It would be great to be able to set some parameters (such as verbosity or rc file) this two way, equivalently. For instance:

oclaunch <some-command> --rc /path/to/rc

would be the same as

export OC_RC=/path/to/rc
oclaunch <some-command>

Closely related to #11, enlarge it by adding the opposite direction (from argument parameter to environment variable when #11 is environment to parameter)

We should document it, adding documentation label.

Added documentation label

Order of parameters should be (ordered from the most significant to the less):

• Command line argument, i.e. --rc.
• Environment variable, i.e. OC_RC.
• Value in the configuration file.
• Preset of the program, i.e. ~/.oclaunch_rc.json.

This needs test.

Added need_test label

mentioned in issue #21

Patdiff maybe interesting to compare with, especially here.

Milestone changed to %v0.3.1