Add formatter_cli

[thomasp85]

Fix #210

In many ways this works like formatter_glue() but with the additional convenience of the cli syntax

[daroczig]

This is great, thanks! I plan to merge this PR as-is after accepting the above minor change, but I would appreciate it if you might have the bandwidth to pick up the related #203 and #204 PRs as well (unfortunately, I lost momentum with those PRs due to the many git conflicts) to standardize all these similar functions.

[thomasp85]

I'll have a look at those PRs - do you want me to mainly go through the conflicts or completely rework them?

[daroczig]

At this point, I think resolving the git conflicts in those 2 PRs and then doing it again after merging one of them is more painful than creating a new one based on those, which handles this for not only 2 (glue and glue_sage), but this newly introduced fn as well.

But I might be wrong and resolving the git conflicts might be easier -- I defer to you if you have time to investigate. If not, I'll try to pick up those myself sometime 😊