fuelup check

[eightfilms]

Closes #16
(note: this merges into #62)

Now that we have DownloadCfg in #62 it's much easier to support fuelup check, since all we need to do is to build the cfg and compare it with the current version running on the machine.

Usage

$ fuelup check
forc - update available: v0.14.5 -> v0.15.1
fuel-core - up to date: v0.8.0
fuelup - up to date: v0.1.2

There is definitely a more elegant way of handling checks - i.e. we shouldn't panic on each command but simply collect the results and print it all at the end. For example, if fuel-core was not installed, fuelup check would panic, but ideally it should do something like this:

Have since updated this PR by not panicking if the component cannot be found/run, instead showing a not found message. I was wondering if there should be an action message here i.e. if any of forc, forc-fmt, forc-explore, or forc-lsp are missing or outdated, should we add a reminder to do fuelup install? Then again it might be intentionally not updated on purpose (if say forc has a breaking change that breaks a dev's project). Due to this reason I decided not to include this action message.

Also included plugins in the check.

$ fuelup check
forc - up to date: v0.15.1
 - forc-fmt - up to date: v0.15.1
 - forc-explore - update available: v0.12.2 -> v0.15.1
 - forc-lsp not found
fuel-core - up to date: v0.8.0
fuelup - up to date: v0.1.2

[adlerjohn]

Maybe add an option that explicitly lists bundled plugins, e.g. --verbose?

[mitchmindtree]

For the most part looking good!

Added a couple comments for now, but might be a little easier to review after landing #73 and merging or rebasing onto it.

[mitchmindtree]