fuelup check: command to check for updates

[eightfilms]

Having a fuelup check command that tells us if we need to update our toolchain would be useful. Although this command is technically not needed (because fuelup install downloads the latest binaries anyway), this command would be helpful in branching out into building other commands like fuelup update (updates all components), fuelup default [stable|nightly] (switch between toolchains), since these commands rely on versioning checks or toolchain checks (which we do not have the concept of yet).

We can store update tags within a dedicated folder in .fuelup, in the format [bin]-[version] (like how our packages are released at the moment):

/.fuelup
    /bin
        forc
        ...
    /binary-versions
        fuel-core-0.8.0

Then we check if the tag version is the same. Simple solution for now until we have toolchain versions in place - then it's also trivial to switch over to checking for the toolchain version

Basic implementation is now at #64 - for now we are running whichever component is first available from path atm. In future there is probably a better way to do this, especially when we have multiple toolchains.