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 &lt;filename&gt; &lt;replace or append&gt; 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.