Fix cli dot-arg-parsing

```
$ st -.

/usr/local/lib/node_modules/st/bin/server.js:29
      if (dot.charAt(0) === '-') {
              ^
TypeError: Object true has no method 'charAt'
    at Object.<anonymous> (/usr/local/lib/node_modules/st/bin/server.js:29:15)
    at Module._compile (module.js:449:26)
    at Object.Module._extensions..js (module.js:467:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Module.runMain (module.js:492:10)
    at process.startup.processNextTick.process._tickCallback (node.js:245:9)
```