Option to modify FFmpeg command

[Knud3]

Advanced options should have a textbox option where users can modify FFmpeg command line. Of course, there will be warning text next to it what warning that modifying values could break the whole export. And "Reset to default" button.

[mifi]

This is a bit tricky, because it's not just one command that is being run when you export, but a number of different ffmpeg commands. What you can do is to go into help, and find the last executed commands. then copy them to your command line, edit them and then run the mamnually.

[vertigo220]

As I mentioned in #126, a solution to this could be to add a word/string replacement option, so instead of having to manually edit every command, users could set it to always replace one word/string with another, e.g. "param_word_a" will always be replaced with "param_word_b" and "command string 1" with "command string 2."