Adopt a labelling scheme for issues

At the moment our issues are a mess, and not very well categorised. We should probably adopt [a labelling scheme like Rust's](https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#issue-triage)!

Plasma has [a nice labelling scheme](https://github.com/PlasmaLang/plasma/labels) with a 'skill' label. This might help folks find something that they can help out with!