Case-insensitive detection of mpegurl type in source (#632)

application/x-mpegURL is legal and often used in the wild.
Embed HLS with correct mime-type (#633)
Avoid HLS desktop Safari crashes (#637)

Conditions for crash:
- hls source
- desktop browser - Safari, not tested with desktop Android
- splash setup
- preload != none

Caused by additional load event when preload != "none".

Solution: extend above conditional to
preload != "none" && video.type != "mpegurl"

The additional load is still triggered on devices where dataload is not
supported, e.g. iPad.
Make HLS seekable (#625), htanks @blacktrash