Tfhd: no way to set default_base_is_moof flag when encoding

[alexkirsz]

Would it be possible to provide a way to set the default_base_is_moof flag on Tfhd? I'm currently working around this by manually patching the encoded bytes after the fact, which is fragile.

[bradh]

Just another flag IIUC. Can you send a PR?

[alexkirsz]

Implemented this in #151