Support property-expansion for command

[natural-harmonia-gropius]

Syntax:
property-expansion

Example:

input.conf

set output TRC to be the same as input.

set target-trc ${video-params/gamma}

ternary operators like.

set target-trc ${?video-params/gamma==pq:hlg}${!video-params/gamma==pq:bt.1886}

evafast like speedup.

SPACE           cycle pause                                                                 #@click
SPACE           no-osd set speed 1; set pause no                                            #@press
SPACE           ignore                                                                      #@release
SPACE           no-osd add speed ${?speed==4.00:0}${!speed==4.00:0.1}; show-text ${speed}   #@repeat

TODO:

• MPV parse the input.conf on startup and shows an error.
  

• Injection via ";" in the string.
  Split commands by ";" and compare the array length of raw_cmd and expanded. This is probably safe enough.

> filename="example; set unsafe true; play another.mp4"
> set title ${title}
> set title example; set unsafe true; play another.mp4

[zhongfly]

第二点：
command_invert里用分号分隔也会有类似问题，命令是set title "example; 6666; 7777"就会分割错误变成{'set title "example','666','777"'}处理第2，3个元素时local property = subs[prefix == "" and 2 or 3]应该就会报错