Proxy to subcommands and be nice as a global install

I think it'll be nice to use `mjs` as an entry point for metascript tools to avoid polluting the global npm space.

    npm install --global meta-script
    cd project
    npm install meta-script mjs-lint
    mjs -v foobar.mjs
    mjs lint foobar.mjs

Any installed NPM package prefixed with `mjs-` can be used as a subcommand, the compiler will simply pass the arguments to its `mjs.js` module which must export a function.

Additionally, now the binary will try to load a local install of the compiler if present. This allows to use different compiler versions for different projects but still use the global `mjs` command.