cargo-check cannot be used with multi-target crate

[W4RH4WK]

cargo check cannot be used directly in a multi-target crate, one has to state which target to build (the library or a selected binary).

Fixing this could get a bit tedious since selecting a target binary will be project specific, not a general option for the linter. But targeting the library can be a global option. Therefore I propose to add another command to the drop-down menu dubbed check --lib, which invokes cargo check --lib.

This way people can at least build the library in a multi-target crate for now.

[colin-kiegel]

link to the upstream issue rsolomo/cargo-check#4