Adds '.go' (for the programming language Go) as default

[hsson]

Simply adds the file extension '.go' as a default. This is the default extension for the programming language Go. I do a lot of programming in Go and I like to use gitinspector - and it's annoying to do -f go every time.

I totally understand if this is too small to include in the project or if you don't think Go belongs as a default in gitinspector - but hey, I wanted to give it a shot :smile:

[adam-waldenberg]

Hi @hsson. Sure. If you want it enabled by default, we also need to add metrics support. See issue #59 and #86 also.