added phantomFlags option

I added a way to pass phantomjs command-line options.

Here is the current list... (from `phantomjs -h`):

```
  --cookies-file=<val>                 Sets the file name to store the persistent cookies
  --config=<val>                       Specifies JSON-formatted configuration file
  --debug=<val>                        Prints additional warning and debug message: 'true' or 'false' (default)
  --disk-cache=<val>                   Enables disk cache: 'true' or 'false' (default)
  --ignore-ssl-errors=<val>            Ignores SSL errors (expired/self-signed certificate errors): 'true' or 'false' (default)
  --load-images=<val>                  Loads all inlined images: 'true' (default) or 'false'
  --local-storage-path=<val>           Specifies the location for offline local storage
  --local-storage-quota=<val>          Sets the maximum size of the offline local storage (in KB)
  --local-to-remote-url-access=<val>   Allows local content to access remote URL: 'true' or 'false' (default)
  --max-disk-cache-size=<val>          Limits the size of the disk cache (in KB)
  --output-encoding=<val>              Sets the encoding for the terminal output, default is 'utf8'
  --remote-debugger-port=<val>         Starts the script in a debug harness and listens on the specified port
  --remote-debugger-autorun=<val>      Runs the script in the debugger immediately: 'true' or 'false' (default)
  --proxy=<val>                        Sets the proxy server, e.g. '--proxy=http://proxy.company.com:8080'
  --proxy-auth=<val>                   Provides authentication information for the proxy, e.g. ''-proxy-auth=username:password'
  --proxy-type=<val>                   Specifies the proxy type, 'http' (default), 'none' (disable completely), or 'socks5'
  --script-encoding=<val>              Sets the encoding used for the starting script, default is 'utf8'
  --web-security=<val>                 Enables web security, 'true' (default) or 'false'
  --ssl-protocol=<val>                 Sets the SSL protocol (supported protocols: 'SSLv3' (default), 'SSLv2', 'TLSv1', 'any')
  --ssl-certificates-path=<val>        Sets the location for custom CA certificates (if none set, uses system default)
  --webdriver=<val>                    Starts in 'Remote WebDriver mode' (embedded GhostDriver): '[[<IP>:]<PORT>]' (default '127.0.0.1:8910')
  --webdriver-logfile=<val>            File where to write the WebDriver's Log (default 'none') (NOTE: needs '--webdriver')
  --webdriver-loglevel=<val>           WebDriver Logging Level: (supported: 'ERROR', 'WARN', 'INFO', 'DEBUG') (default 'INFO') (NOTE: needs '--webdriver')
  --webdriver-selenium-grid-hub=<val>  URL to the Selenium Grid HUB: 'URL_TO_HUB' (default 'none') (NOTE: needs '--webdriver')
```