Allow plugins to inject cookies into browser session

[gpoul]

Allowing plugins to inject cookies into browser sessions would allow many scenarios where tokens or IDs should be available in the browser session and sent to the server, but should not appear in every test case, but be added transparently to the test.

This would be especially useful for tagging requests as test-requests, to influence which version to target from a load balance, to provide API keys, or authentication tokens.

wd seems to have the necessary code to enable this: https://github.com/admc/wd/blob/31981c6db7e284011281902592c281375097ac79/lib/commands.js#L1980