Feature Request: Allow subsets of requirements

[tabrindle]

Consider a project with an optional dependency based on what scripts are run, or who is running them.
In some cases, you may want to block a script with a non zero exit code from solidarity failing a check in one script, but not another. Some of these cases might be achieved by different solidarity files, but this could be unsustainable if there are many different sets of rules to run.

Example: you want to check for a certain git author email before commit, but that rule shouldn't block someone from installing or building the project.

Second example: you have an optional dependency on docker, but only certain developers will need that for certain tasks. Docker would be unnecessary for the rest of the team.

Third example: Having a different set of requirements for ci

This could be passed via a --set option like so:

solidarity --set development || solidarity --set testing || `solidarity