custom ffmpeg path

[elbadryd]

Checklist

• [X ] Code changes are only for the relevant bug fix or feature
• [X ] New code lints (via npm run lint) without any errors or warnings
• [X ] The corresponding issue is [FEATURE REQUEST] Add option to use custom ffmpeg path #61

Description

Adds ffmpegPath variable as part of config object to use custom ffmpeg path.

[tungs]

Thanks for your interest @elbadryd! It looks like a useful feature, and I merged it into the main branch. It should be released in the next version.