Support spec files

[Ana06]

addlicense appears not to add headers to .spec files such as https://github.com/mandiant/capa/blob/master/.github/pyinstaller/pyinstaller.spec

[flwyd]

It looks like that file is Python code with special syntax. I wonder if there are other file formats using .spec as an extension; it seems like something that multiple programs might use. I see that RPM has a spec format, though it's not clear if it uses that extension, and it uses a # comment anyway. A spec extension is also mentioned here.

Feel free to send a pull request and we can investigate further.