Implement --as-root option.

 - The option allows to overwrite defaults for using sudo with specific
   installers from the command line.
 - The only installers currently not using sudo by default are homebrew and cygwin.

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 - This needs some testing, possibly unit tests.
 - Should we change the name of the option?
You can do `(cmd if type(cmd) == list else [cmd])` as just `list(cmd)` I think.