Support both the mp4: and .mp4 syntax with RTMP

[jeroen]

When streaming over RTMP, some people use the "actionscript" syntax in setting MP3 / MP4 streams:

file = "mp3:mysong" file = "mp4:myvideo"

The player always appended these automatically, given a known extension was found:

file = "mysong.mp3" file = "myvideo.mov" / "myvideo.mp4" / "myvideo.f4v" / "myvideo.aac" / "myvideo.m4v"

By checking for an ":" syntax, we can support both ways, so the stream always works.