Add possibility to set severity of problem markers

[pulse00]

Right now the plugin creates error markers by default when some tests fail. This also happens, when something is not setup properly by the user or MakeGood itself but the "real" tests don't fail, (see #14).

The only way to get rid of those errors at the moment is to uninstall the plugin completely. Adding the possibility to set the severity for MakeGood problem markers in the preferences would solve this, e.g. something like Error, Warning and Ignore.