Feature extend config syntax Issue #278

Changed `browsers` configuration syntax as suggested in https://github.com/testacular/testacular/issues/278#issuecomment-11986383 except that I didn't know what to do with the `headless:` key proposed there. The list of args, if provided, completely replaces the default args normally used by that launcher. Within the arg list, occurrences of `$TEMPDIR` and `$URL` are replaced by the temporary dir and the URL passed in to browser.start, respectively. There is also a `cmd:` for specifying the path to the executable.

So `browsers` can now be:
```javascript
browsers = [{
  name: 'Chrome',
  args: ['--user-data-dir=$TEMPDIR', '--start-maximized', '$URL'],
  cmd: '/usr/local/bin/chrome'
 }, {
  name: 'Firefox'
 }];
```