Simplified rendering of YAML_TEMPLATE.

There used to be some format spec replacement in yaml.py, then a string
replacement in vid. Now, there is only format spec replacement in vid.
Added test coverage for YAMLReader._check_filter().

Corrected a few bugs. Changed a format string to a string concatenation
because some dict representation could be mistaken for a format spec.
Increased test coverage of YAMLReader.

Changed some ValueError into TypeError where better appropriate.
Added test of _check_globals(). Improved code and docstring.
Added test of _check_music().
Increased test coverage of YAMLReader.

Added tests for _check_multiplexer().
Separated shot specific checks from _check_movie() to _check_shot().
Added tests for _check_shot().
Added tests for _check_movie().
Added test for YAMLReader()._check_root()
Added test for YAMLReader().load().