style: fix audit --online

This passed a symbol to `:except_cops` which caused a .

style: don't require rubocop until installed.

This avoids an error when RuboCop installed installed yet.

CC @GauthamGoli FYI.

Remove `existing` from PATH when adding `Gem.bindir`.

audit: Allow skipping/selective running of cops and cops refactor

Install a bottle from an URL

caveats: tweak keg_only style

Install etc/var files on postinstall.

utils: create GEM_HOME when installing Gems.

Also, don't delete them after that. This means that `brew postinstall`
becomes a way to easily reinstall configuration files for any formula
without needing any changes to any bottles or requiring a reinstall.

It may not exist before Gem installation which means that the resulting
installed gem will not be found in the PATH.

Refactor PATH generation.

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

caveats: tweak keg_only wording

Current:
```
This formula is keg-only, which means it was not symlinked into /usr/local.

Qt has CMake issues when linked
```

After:
```
This formula is keg-only, which means it was not symlinked into /usr/local,
because Qt has CMake issues when linked.
```

Revert "Fix `IO#select` blocking."