Unify cmd and cmd w/ version

[rhagenson]

Because I just sank more time than I should have (reading the docs fixed my issue obviously) can we unify the two types of update to the following form:

Install latest version

ponyup update ponyc release

Install specific version

ponyup update ponyc release 0.33.1

This is a change from how currently the correct way to do it is to replace release with release-0.33.1 rather than adding an additional piece of information in the form of 0.33.1 -- it seems to me it should be ponyup <cmd> <tool> <channel> [<version>]

This change would then be best extended to ponyup select as well to make it ponyup select ponyc release 0.33.1

[SeanTAllen]

This would be a breaking change and the PR should get a CHANGELOG- changed entry.