The cargo updateVersion preset should allow a targetDir to be specified

[lurch]

Spin-off from a conversation in balena-io/reconfix#40

A project's Cargo.toml (and optional Cargo.lock) may not always be in the project's root directory. The cargo updateVersion preset should allow an optional targetDir to be specified, and it would then attempt to update targetDir/Cargo.toml (and targetDir/Cargo.lock) instead of cwd/Cargo.toml (and cwd/Cargo.lock).

[lurch]

Given his work in this area recently, I'm going to cheekily assign this directly to @majorz :)