Review: WebView tests and Command Line Arguments [fixes #4]

This adds an options object as second argument to the main function that can take an `args` dictionary and a custom `exec_path` string.

```
// test.js
require('node-thrust')(function(err, api) {
  // ...
}), {
  args: {
    "show-fps-counter": true
  },
  exec_path: '...'
});
```