I came to write this plug-in, while solving an “issue” of me, partially (only) explained there:
Variables in command palette.
I won’t comment in this post, as the docstrings already do. If you feel to have comments or feel to see something bad, you can tell and give your rationals.
Just discovered there already exists something similar, since 2012: packagecontrol.io/packages/External%20Command
Will have a look at it to see if that differs a lot or not.
— Edit —
It differs enough. I will mention this other package in the README file and add wordings about the differences, so people can choose what match the best for their needs.