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?
I don't think so:
```
In [11]: list("foo")
Out[11]: ['f', 'o', 'o']
```
I agree that line is kinda ugly code. I will change to not even handle `cmd` not being a list and make sure all callers pass a list to reduce the confusion.