dict: add av_dict_parse_string()

Can be used to set multiple key/value pairs from a string.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
libx264: introduce -x264-params private option

It is a shortcut to set all the params using x264_param_parse,
makes simpler importing settings from other software using x264.
doc: document libx264 options and mappings