add simple repl if no arguments are passed

with this update, alonzo can handle the following:

    $ ./dist/build/alonzo/alonzo
    Lisp>>> (cons 2 3)                 // (2.3)
    Lisp>>> (cons 'this '())           // (this)
    Lisp>>> (cons this '())            // Unrecognized special form: this
    Lisp>>> quit