add an 'include' option as an inverted exclude

[jupahe64]

I felt like this was missing and it was the reason why I searched for a tool like this in the first place. (I wanted to only show files with a certain extension) I have never really used rust but I felt confident enough to do this, if I made an obvious mistake please tell me. Also feel free to change the name of the option, it's just the first that came into my mind.