Handle poor media conditions for user with SampleBuilder

[Sean-Der]

See comment by @tmatth pion/example-webrtc-applications#130 (comment)

Right now the SampleBuilder will lead to poorly generated files in bad conditions. We should provide a better API for users.

[jech]

Related to jech/galene#206, even though Galene uses a different samplebuilder implementation. It's on my todo list, but not at the very top.