Support absolute path for --file and --options.

This patch allows `--file` and `--options` to take absolute file path.

Without it, when given an absolute path, colorguard currently concatenates `cwd` and the absolute path, and throw an error.

```
$ colorguard --file ~/foo.css

fs.js:427
  return binding.open(pathModule._makeLong(path), stringToFlags(flags), mode);
                 ^
Error: ENOENT, no such file or directory '/Users/shuhei/Users/shuhei/foo.css'
    at Object.fs.openSync (fs.js:427:18)
    at Object.fs.readFileSync (fs.js:284:15)
    at Object.<anonymous> (/Users/shuhei/.nvm/v0.10.28/lib/node_modules/colorguard/bin/colorguard:16:12)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)
    at startup (node.js:119:16)
    at node.js:906:3
```