Pass list of detected browsers to plugins file via config object

[bahmutov]

Currently in v3.4.1 we detect browsers on the user's system, then load the cypress/plugins/index.js file, but the config object we pass does NOT have the list of browsers. By passing the list of browsers via config we can let the user check if a required browser has been detected for example.

Second goal:
allow plugins to return the config object with modified browsers list and then use it.