Support installing the latest release of a given major version

[patrickfreed]

Most commands accept a toolchain selector argument to identify the toolchain(s) the command is intended to operate over. Selectors for stable releases can currently be of the form a, a.b, or a.b.c. The install command requires that the user specify at least a minor version (e.g. a.b or a.b.c). We should update it to also accept just the major version (e.g. swiftly install 5 would install the latest Swift 5 release).