and add explicit version when not preexisting

--version=1.2.3 option to set `version "1.2.3"`

Note that --version=0 will remove a no-longer-needed explicit version

info: print requirements

RuboCop: MacOS

add pcmpl-homebrew for homebrew emacs plugin

Speed up `parallel_tests` by splitting integration tests.

Fix documentation for --caskroom default

fixtures: add test.m4a

doctor: remove outdated Homebrew check.

ARGV: kegs: error on empty arguments

test.m4a contains an AAC LC stream inside an M4A (MPEG-4 Part 14)
container. It is generated by FFmpeg 3.1.3 from test.wav:

    ffmpeg -y -i test.wav -c:a aac -profile:a aac_low -b:a 32k test.m4a

test.m4a facilitates testing utilities or libraries that work with the
AAC codec or the M4A container, e.g., alac.

update.sh: don't restore branch to stable.

This is no longer relevant in the bright, new tagged future.

Fixes #1151.

Only run for Homebrew developers so assume they don’t want to switch
back to an old stable branch.

Fixes #1141.

update.sh: use HOMEBREW_UPDATE_TO_TAG.

This makes the code easier to follow rather than having to know
HOMEBREW_NO_UPDATE_CLEANUP implies not updating to a tag.

Fix `search_tap` regex/string matching.

Fixes #1138.

download_strategy: never verbosely untar

The verbose mode of tar is too pointlessly verbose.

RuboCop: Style/CaseEquality

Alter mkdir wrapper use mkdir_p to create intermediate directories

update.sh: use GitHub tags API.

Otherwise if we've committed to `master` and someone `brew update`s
before we cut the tag then they won't be updated to the latest version.

RuboCop: Style/AccessorMethodName

Update manpage.

Add some missed changes. I'll add a CI check for this at a future point.