[Feature] Support Brew Gem

[derimagia]

Brew Gem, for managing Gems through Brew:
https://github.com/sportngin/brew-gem

Right now after installing a pip from brew-gem, it won't add to the Brewfile. When cleaning up, the gem will be removed from the system since it isn't in the Brewfile.

[rcmdnk]

This is also fixed at 3.7.9.

On the other hand, though it could be beyond this issue,
brew-gem uses gem names as is,
which sometime conflict with original formula in Homebrew (such git).
And there are no way to search for such brew-gem installed one.

I made a pull request for this issue.
Once it is fixed, I will separate a management of brew-gem packages like it is doing for brew-pip.

[rcmdnk]

FYI.

brew-gem was updated to put gem- prefix to the package name.
If you install heroku by brew-gem like:

$ brew gem install heroku

then, you will have gem-heroku in brew list.

brew-file has been updated to check such name,
and will make a list of gem, like:

gem heroku

in Brewfile, instead of brew gem-heroku.

With this update, you become to able to use such brew file install with such gem- packages.