Added option to manually start the browser.

[HerrLapsus]

To work with cloud services like browserstack it can be beneficial to control when the session is started and therefore start the session manually.

One case in which this is useful is issue #190. You can set the desiredCapability.name and then start the session by executing this.helpers['WebDriverIO']._startBrowser()

As the manual option is set to false by default the behavior doesn't change.

[DavertMik]

Thanks for the PR. I like the implementation but I don't like naming, probably they should be more explicit about the problem it solves. manualStart looks like it expects you to launch a browser locally and wait the Selenium to handle its session (somehow).

Maybe it should be named like reuseSession or smth else... With documentation for potential use case at cloud service providers

[DavertMik]

@cnworks when I merge your PR could you help the project a bit more. I was thinking it would be useful to have dedicated "Cloud Testing" guide in documentation. Could you write some paragraphs on it, to share your experience?