Feature: run custom commands

[hypnoglow]

Implement a feature allowing users to define custom commands in the config file, and run them on installing dotfiles.

[flowchartsman]

I'm 90% of the way through this. flowchartsman@ba71fe9

Need to add some kind of support for Windows, whatever that might be, and clean up a little and then I plan to start on #3

[hypnoglow]

@flowchartsman Awesome work, thanks!

I would also like to give some love to this tool and dip my fingers in the code when I can find time 🙂