Update nomenclature for headless / headed to run / interactive

[brian-mann]

Per this issue: cypress-io/cypress#1140

[jennifer-shehane]

In Cypress there are only two modes of operation:

• Interactive Mode (cypress open) - where you write your tests and run a single or handful of tests. You can use snapshots and the process never exits. This is what you'll do during day to day development.
• Run Mode (cypress run) - where you run your tests to completion and receive an exit code with the number of failures. In this mode - the browser may be spawned headlessly or headed. This mode is used when you want to run all of your tests - typically in CI.

The terms headless and headed are specific to the way a browser is spawned - and don't have much meaningful value to Cypress. Cypress doesn't care which way it's spawned - the same things will happen headlessly or headed.

The only difference today is that we only record videos when running Electron in headless mode. Recording videos of headed browsers is of course possible - and we already bundle ffmpeg to do this - but we haven't yet implemented that behavior.