Add option to delete local version separate from --force

[anitgandhi]

This tool is great as a Go version manager, but it would be nice if there was an option to delete an existing installation/version of Go.

For example, I just installed 1.10.2 via gimme 1.10.2, and I have source ~/.gimme/envs/latest.env so it picked it up just fine; however I have my previous Go installation still around. Using the --force option is useful, but sometimes you just want to have multiple around, until you don't.

Usage could look like gimme --delete [go-version]

This could be pretty trivial to add since there's already a _wipe_version function.