- only document HOMEBREW_* variables in the manpage (although still
  read from all the original environment variables).
- resort manpage environment variables in alphabetical order
- check the original path for trailing slashes

Also refactor audit cops into two "departments"
 - FormulaAudit
 - FormulaAuditStrict

GitHub provides a description and homepage field so let `brew create`
use them where possible. Also, detect GitHub repositories based on
`releases` as well as `archive`s.

Add `--only` and `--except` methods which can be used to selectively
enable or disable audit groups.

This is useful when you're pulling PRs where individual bottles can't be
uploaded/built but you still want to pull the PR as a whole anyway.

Closes #2186.

Includes replacing some non-ASCII characters, which weren't being
processed properly.

Also update command specifications to match descriptions.

As requested in https://github.com/Homebrew/homebrew-core/issues/9316.

These can now be done with just `brew upgrade`, like other upgrades.

Fixes #1818.

Fixes #1817.

Unfortunately `brew linkapps` cannot behave nicely with e.g. Spotlight
using either aliases or symlinks and Homebrew formulae do not build
"proper" `.app` bundles that can be relocated. Instead, please consider
using `brew cask` and migrate formulae using `.app`s to casks.

GitHub now nicely generates a documentation site for us at
http://brew.sh/brew based on our docs folder. Optimise the output of
this and the GitHub docs directory for readability and the various user
groupings.

When reproducing issues with software that hasn’t been bottled yet on
your version of macOS it can sometimes be helpful to use `or_later`
bottle functionality i.e. just use the bottle for the latest version of
macOS available. This maps well to the existing `--force-bottle`
argument so it will now act as if the latest bottle has a `or_later`
ending.