add option to check for module dependencies/conflicts

[xorpaul]

Idea:

If a setting, like check-dependencies is set inside a Puppetfile.

Then parse all metadata.json files and check if a compatible or conflicting module is found.

Report and stop the g10k run at the end with an exit code 1.

If the user wants to force the g10k run anyway he/she has to remove the setting inside the Puppetfile and retrigger the g10k run.