Add Sauce performance enablement and disablement

[nadvolod]

New feature released from Sauce https://wiki.saucelabs.com/display/DOCSDEV/Measure+Page+Load+Performance+Using+Test+Automation It's relatively easy to implement and allows a user to pause performance until a certain page.

browser.execute('sauce:performanceDisable')
browser.url('https://www.json.org/json-en.html')
browser.execute('sauce:performanceEnable')
browser.url('https://example.com')