Add ability to specify file format

[pareeohnos]

I'm writing an app which involves validating a user defined yaml file, however this file doesn't have a .yml extension. As a result, pajv is raising an exception about it having an unknown format.

The only workaround I can come up with at the moment, is to rename the file during processing to include the .yml extension but it feels like a major hack. It would be great if I could pass in a format option to specify that the file is yaml rather than only relying on the files extension.