Make release prefix configurable

[sunshowers]

Hi there! Thanks for writing self_update!

I'm playing around with it and I noticed that the release prefix is currently not configurable: https://docs.rs/self_update/0.27.0/src/self_update/backends/github.rs.html#55. Versions beginning with v are not valid semver, and using v is more of a convention than a hard rule. Other projects might use their own prefixes in their tag names (e.g. my-package-1.3.4, common in monorepo situations where multiple binaries are released), or no prefix at all (e.g. 1.3.4).

Moreover, if tag names begin with v but don't use the v1.3.4 naming scheme, they will have unexpected results. For example, if a tag is called very-cool-package-1.3.4, its version will be returned as ery-cool-package-1.3.4. This seems non-ideal.

My proposed solution would be to add a configuration, e.g. fn tag_prefix(&str). This can be "v" by default, but could also be set to "my-package-" or even "". You could then even filter out tags that don't match the prefix.

This would be a breaking change.