add --robot-paths configure option

@geimer: this will allow you to add the easyconfigs archive/repo path to the list of robot paths, without always having the robot enabled.

We can consider including the easyconfigs repo path by default for EasyBuild v2.0, people can easily kick it out by customising `--robot-paths` in their EB configuration (although we would need a way to indicate the path to the installed easyconfigs package then without hardcoding it, hmm...).

This needs further testing and some enhanced unit tests, but it should be working.
what does `priority` mean here? `robot` values are preprended or does it overwrite the `robot-path` values?