Add more version specifiers

[ambergamefam]

While this will probably be fixed pretty soon, I just ran into a critical issue with Knit v1.5.2, and need to revert the version to sleitnick/knit<=v1.5.1

Wally only supports the "@" version specifiers, which gets the latest version so long as the major version hasn't changed (according to the readme saying it's equivalent to NPM's ^ specifier).

However, in critical situations like this, I am leaving my project in an unstable place, with an inability to revert to an old version through wally (have to do so manually, and this doesn't scale well on a team).

It would be nice to have version specifiers which cap your supported version on a specific library.