js: add configuration option on startup

• src/index.js: Added an optional configuration object for the startup.

• configuration: timeout - the initial check for a connection ran into timeouts on slower testing machines. 500ms for initial startup is not sufficient everywhere. The default timeout was raised to 1000ms, and as an option this timeout can be increased even further.

• BrowsertestExtension: Set the initial connection timeouts to 2 seconds, to be able to test on slower machines.