provide per-file-options for loadfile command

mpv provides per-file-options when open a file from command line, but there's no equivalent thing for 'loadfile' command when open a file using libmpv or slave mode.
This patch adds an optional parameter which repsents option list of key/value pairs for loadfile command.
For instance, to start a file from 30sec, call 'loadfile <filename> <replace or append> start=30'.

I'm not sure I'm doing this in right way because I don't fully understand mpv's property/command system so please review the codes.