This PR extends the `autoplay` to the player with a few options that should hopefully make working with browsers that disable unmuted autoplay by default easier.
The current boolean option will match current behavior and any unknown option will be treated as it does now. The new options are the string values `muted`, `play`, and `any`.
- `muted` will mute the element and call `play()` on `loadstart`,
- `play` will call `play()` on `loadstart()`, this is similar to the `autoplay` attribute
- `any` will call `play()` on `loadstart()` but if it fails it will try muting the video and calling `play()` again.
The option for the player techCanOverridePoster is introduced in this commit. It allows techs to update the post whenever they like. isPosterFromTech_ is introduced as a private player field in order to track when a poster was set by a tech. This allows us to clear the poster whenever the tech is disposed of by the player.
Additionally, attempting to set the same poster more than once will have no effect / no changes will be made, since the poster is the same. This was done in order to stop triggering multiple posterchange events when calling player.poster(aPoster) with techCanOverridePoster set to true.
When a tech is disposed and a poster was set by it, unset the poster.
Pass a `canOverridePoster` option to techs to know whether techCanOverridePoster was set.
Fixes#4910.
A possessive belonging to an "it" doesn't need an apostrophe. Don't believe me? Ask [the Oatmeal](http://theoatmeal.com/comics/apostrophe) (look for the velociraptor)!
