Attempt at defmain.  It's close to being the right thing...

This works right if you do something like:

```
  (defmain [&rest args]
    (print "now we're having a fun time!")
    (print args))
```

Which outputs:

```
  $ hy test.hy
  now we're having a fun time!
  (['test.hy'],)
```

Which is expected.  But if you try to import the file, there is no
"main" function to be found.  Presumably because it's wrapped in the
"do" s-expression.