Implement `uninstall` command on Linux

[patrickfreed]

The uninstall command accepts a toolchain selector and uninstalls all matching toolchain(s) after prompting for confirmation. The skeleton code for this command is already implemented, but the following need to be completed in the Linux platform implementation:

• Platform.listToolchains for getting the list of currently installed toolchains
  • This can probably be default-implemented on the Platform protocol, since all the information is coming from config.json anyways
• Platform.uninstall: basically just deletes the toolchains/<toolchain name> directory and removes it from config.json