Add a log levels and history api: `videojs.log.level()` and `videojs.log.history()`.
`.level()` will return the current level and you can also set it to be one of: `all`, `error`, `off`, or `warn`.
`.history()` will return a list of all things logged since videojs loaded. It can be disabled via `videojs.log.history.disable()` (and re-enabled with `enable()`) as well as cleared with `videojs.log.history.clear()`.
