document how to use ignore file types in configuration

I think that ideally the command-line parameter and the config file parameter for the type of ignore file to create should be the same, or at least there should be some spelling that's accepted for both, but perhaps there's a reason I haven't seen for this not being the case.

This commit just changes the documentation for the command-line tool to mention the difference and include an example of how to specify an ignore file type in the configuration, and thereby fixes issue https://github.com/xsawyerx/module-starter/issues/17 (which I found when googling for the problem I had). If you'd rather I fix it by changing the code to accept one or both of the spellings in both places, I'll do that instead.