Provide browser-specific APIs for some (or all?) specific parameters

[mercmobily]

At the moment, if you want to add plugins in Chromedriver, you need to pass an array of base-64 packed plugins. This is a little much... but we need to decide whether we want to add a specific call per browser-specific parameter, or maybe only for the "nasty" ones.

[mercmobily]

The infrastructure for it is there. The theoretical steps now would be:

• Figure out the "proprietary" functions that browsers provide
• Figure out if those proprietary functions will be available in the longer term (probably a ticket with the makers would be good)
• Create one issue per browser, detailing the list of actions to add with a checklist and link to docs

At that point, implementing them (along with writing tests) should be pretty straightforward.

The question is: do we want to go that way? Or, should this driver JUST cover what the w3c dictates? (which is meant to be the point of it in the first place...)