Add support for optional args in config/config.yml for Watir Webdriver

Hello,

I'm using Taza with RSpec in my project and I faced the need to start PhantomJS via Watir Webdriver with arguments (see http://stackoverflow.com/questions/26583409/watir-webdriver-phantomjs-and-urls-that-redirect-to-https).

I tried to patch Taza::Browser.create_watir_webdriver method, but it seemed to be overwritten later, so I did the following:

*config/config.yml*
```---
driver:   watir_webdriver_ext
browser:  phantomjs
options:
# See Selenium::WebDriver.for
# :profile: ...
# :port:    ...
 :args: [--ignore-ssl-errors=true, --ssl-protocol=any, --debug=true]
 :port: 8888
```

*spec/support/patch_taza_browser.rb*
```...
def self.create_watir_webdriver_ext(params)
  if (not params[:options].nil?) && (not params[:options].empty?) then
    Watir::Browser.new params[:browser], params[:options]
  else
    Watir::Browser.new params[:browser]
  end
end
```

I knew that I could pass in an instance of Watir::Browser created beforehand with all optional arguments I wanted, but it would be inconvenient in my case.

In the proposed pull request I've modified Taza::Browser.create_watir_webdriver method. I've got a faint feeling that I may be reinventing a wheel, and if yes, how could I do that in the other way?
What happens if you send options such as `{}`? If it works, you can rewrite it as something like `Watir::Browser.new(params[:browser], (params[:options] || {}))`