Warn when conflicting rules enabled

[mivok]

Add a 'conflicts_with' attribute to rules, and show a warning if conflicting rules are enabled (Make it a fatal error, but allow it to be overridden with a command line option).

There should probably also be some dedup logic in there, so if MD123 conflicts with MD124, and MD124 conflicts with MD123 (because I was diligent and put the conflicts_with entry on both rules), then the error message doesn't show up twice.

[cirosantilli]

Another possible solution: #47

[mivok]

Parameterized rules from #47 are now implemented, and so this will no longer be necessary.