Scripting: Support #! "shebang" on first line

Currently, executing:

```haxe
#!/usr/bin/env haxe --interp

class Main {
static function main()
trace("hi");
}
```

results in:

```
~> ./Main.hx
Error: Could not process argument ./Main.hx
Package name must start with a lower case character
```

If --interp would understand shebang lines, we could use Haxe for scripting. :)