Add prefer-local semantics and volta use command

[charlespierce]

Related to #502 and from some discussions with @thoov, it would be good to have a flag --prefer-local for volta install, which will attempt to resolve the semver version you are giving with the locally available versions, and only access the internet if that fails.

With that functionality, we should also add a volta use command that exists as an alias of volta install --prefer-local, so the typical use-case of switching between local versions of Node could be easily handled with volta use node@10 and volta use node@8.

[charlespierce]

Note: There are some ambiguities around what the user means when they type volta use node@latest: Do they want the latest version overall, or the latest version on their machine? With the --prefer-local flag, the intent seems fairly clear that it would make sense to look for the latest on the machine, but with volta use as an alias, it's less obvious.

From some discussion on Discord, the fact that we will be clearly labelling volta use as an alias for volta install --prefer-local makes it lean towards resolving relative to the user's machine (i.e. volta use node@latest would use the latest node available on the user's local machine) in this case, but an important point is that we need to clearly and explicitly document that behavior, and provide guidance on how to achieve the "latest on the internet" case, so that users who are looking for that behavior can easily find how to achieve their goals.