Add -map option

[jeongmallro]

Hello, @bramp, @Euklios

I added support for the -map option.
This option allows users to control which input streams are included in the output.

Since FFmpeg allows multiple -input options, adding support for -map would be helpful for specifying exactly which streams should be included in the output.

Examples:

-map 0
-map 1:v
-map 0:v:0

I also wonder if you have any plans to support per-stream settings related to #239

Thank you!