installer command accepts -applyChoiceChangesXML option to change
customize options on the GUI installer from the commandline.
(`man installer` for more detailed information)

The introduced option `choice` enables the choice changes to be supplied
via pkg stanza without tricks in preflight code.

Also changes exceptions.rb to use Formatter.url

e.g. this changes:

```
To re-install google-chrome, run:
  brew cask uninstall --force google-chrome && brew cask install google-chrome
```

To:

```
To re-install google-chrome, run:
  brew cask reinstall google-chrome
```

...and update man pages where applicable

Open the incomplete download in append mode instead of write mode.
Opening in write mode truncates the existing file, so curl keeps
restarting downloads instead of resuming the incomplete downloads.

Use instance methods to DRY things up, and fix a bug discovered in
caskroom/homebrew-cask#26067 where Casks with language blocks would
always fail audit.

- Stop supporting archaic "user-repo/token" syntax
- Move regex for parsing tapped Cask token to tap_constants