Add --extra option for extra packages in `requirements.txt` file

[Temerold]

By using the --extra option, users can add custom packages to the requirements.txt file, like so: $ pipreqs --extra pipreqs,example This command generates the correct file, alphabetized and all, including pipreqs and example (in this case).

This adds pipreqs itself and example to the requirements.txt file, as my original intention was to add pipreqs to the requirements.txt file. (I know there are severely easier ways to do so, but I implemented it anyway.)

I added the test_extra_module test, which it passed alongside all other tests; should be good to go!

[Temerold]

this would be great addition

Thank you!

This repo seems however to be very inactive, and I'm thinking about forking and making my own pip package at this point....