Signaling to the user when the tool is called on an incorrect file type

[FLKR-redo]

I have included the tool in a pipeline without realizing I was calling it on a yaml file without adding the --extensions yaml option (ex. j2lint wrongfile.yaml). Even when running in verbose mode the tool doesn't signal that this error has occurred, so I propose to have it error out when such a mistake happens.

Something like:

$ j2lint wrongfile.yaml

ERROR: 'wrongfile.yaml' is not in extensions list. Add it with '--extensions yaml'