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.

-----

 - This needs some testing, possibly unit tests.
 - Should we change the name of the option?
You're right, I did this naive check:

```python
>>> a = "b"
>>> list(a)
["b"]
```

Silly me.