Implement `update` command on Linux

[patrickfreed]

The update command accepts a selector and updates the newest matching toolchain to the newest available matching toolchain, if any. This happens in a few steps:

• List installed toolchains to see if any match the selector
  • If no selector is provided, match the currently in-use toolchain
• If one is found, list available toolchains, and see if any matches the selector and is newer than the found toolchain
• Uninstall the found toolchain
• Install the newer toolchain
  • If the updated toolchain was in use, mark the new one as in use

Updating depends on the functionality from a bunch of other commands, so they will need to be done first (#2, #4).