ver 0.5.0-SNAPSHOT
Allow to install a specific version by `shly install --version v0.x.x'.

You can see available versions by `shly available-versions'.

Obviously, older versions than v0.5.x don't have this feature, so you
downgrade Shelly to such version once, you can't upgrade it by this
option.
Also allow --version :latest to install the latest version.
Update bin/shly.
Fix an :import-from symbol.
Refactor by using conditions.
Decide which shlyfile to load in Lisp.
Change --help to show available commands which are defined in shlyfile.
Update bin/shly.
Also allow to specify a version "latest" as a string.
ver 0.5.0