Support upgrading Cygwin packages

[tylerszabo]

Cygwin setup supports --upgrade-also (see https://cygwin.com/faq/faq.html#faq.setup.cli) which allows Cygwin to perform an unattended upgrade of all installed packages.

choco upgrade -s=cygwin all

Related to #1355

[ferventcoder]

Thanks for the issue and the contribution! We'll take a look at this in more depth hopefully in the next few weeks.

[bcurran3]

If this moves forward, consider also Python's PIP. (Will create as issue in the future if/when relevant.)

[tylerszabo]

I'd like to reconsider this feature; the PR is still viable.