Add atom language plugin to docs

Refactor `shutup`.

add .editorconfig

build: permit weak imports in Go reverse deps.

Make "brew man" also generate Cask's manpage.

Go makes extensive use of weak imports so we need to allow them when
building Go-using software.

Closes https://github.com/Homebrew/homebrew-core/issues/4047.

Also, check in this manpage, adjust the Rakefile and remove the old one.

Fixes #731.

bump-formula-pr: return to starting branch

bump-formula-pr now creates non-tracking branches

Remove LOAD_PATH warning from Hbc::CLI::Doctor.

Install Cask’s default tap if untapped.

Use dummy containers for cask tests.

Remove Cask developer scripts.

After branching, bumping, pushing, and pr-ing; return to whatever branch
was originally checked out.

In most cases, I'd imagine users to want to continue receiving tap
updates from master. However, after using bump-formula-pr, the tap in
which the formula was bumped is left on the working branch that was
doing the bumping and pull-request. After opening the PR, we should
return to whatever branch the user originally had checked out – most
likely master. (But git allows us to just say "previous branch" by using
`-`)

Add permit_weak_imports directive

Formula-Cookbook: document head branch default.

Without `--no-track`, some git setups may automatically set
`origin/master` as the tracked upstream for the newly created branch.
This upstream is what hub defaults as --head when opening PRs. By not
allowing git to set `origin/master` as upstream, hub can then use the
proper --head for the PR.

Since hub still needs to know what --head is intended to be, we can set
the branch's upstream when pushing: with the --set-upstream option.

Fixes #755

`--HEAD` defaults to `:branch => "master"` so document that.

Closes #722.

diagnostic: ignore duplicated `brew-cask` commands

Bottles: bottle revision is now called rebuild.

download_strategy: check if commit nil

These are intentionally duplicated for now until the Cask migration is
complete.

Move Cask core code to Homebrew

Forgot this in #731.

Rename bottle's revision to rebuild.