Allow # in readtable

The command line script reads stxcase.js with a user-provided readtable which makes it impossible to define a # behaviour and use it from the `sjs` command.

Example:
```readtable.js
var sweetjs = require('sweet.js');
var recursed = false;
module.exports = sweetjs.currentReadtable().extend({
    '#': function (ch, reader, toks, source, index) {
        if (recursed) return null;
        var start = reader.index;
        var tok;
        recursed = true;
        reader.readToken(); // read the #
        tok = reader.suppressReadError(reader.readDelimiter);
        recursed = false;
        if (tok !== null && tok.value == '{}') {
            // looks like #{ } form
            return null;
        }
        return [reader.makePunctuator('+', { start: start })];
    }
});
```

```a.js
macro m {
  case {_ $x } => {
    var y = makeValue(42, #{$x});
    return [y]
  }
}
m foo # 5
```

Invocation:
`bin/sjs -l ./readtable.js -s < ./a.js`