How to override "-f rawvideo"?
<!-- You must use the issue template below -->

**Describe The Problem:**
Although I specify `-f mjpeg` in my config, the log shows `-f rawvideo` gets used.

**To Reproduce:**
Add `-f mjpeg` to the source parameter in the config and start a video. The log then shows how ffmpeg contains the `-f` option twice: `ffmpeg -f mjpeg -i http://192.168.178.14/live -an -sn -dn -codec:v libx264 -pix_fmt yuv420p -color_range mpeg -r 15 -f rawvideo`

**Config:**
```
                {
                    "name": "Cam",
                    "motion": true,
                    "switches": true,
                    "videoConfig": {
                        "source": "-f mjpeg -i http://192.168.178.14/live",
                        "stillImageSource": "-i http://192.168.178.14/photo",
                        "maxWidth": 1920,
                        "maxHeight": 1080,
                        "maxFPS": 15,
                        "packetSize": 188,
                        "maxBitrate": 5000,
                        "debug": true
                    }
                }
```

**Logs:**

```
[11/13/2020, 03:22:41] [Camera FFmpeg] [Cam] Video stream requested: 1280 x 720, 30 fps, 299 kbps
[11/13/2020, 03:22:41] [Camera FFmpeg] [Cam] Starting video stream: 1280 x 720, 15 fps, 299 kbps
[11/13/2020, 03:22:41] [Camera FFmpeg] [Cam] Stream command: /usr/local/lib/node_modules/homebridge-camera-ffmpeg/node_modules/ffmpeg-for-homebridge/ffmpeg -f mjpeg -i http://192.168.178.14/live -an -sn -dn -codec:v libx264 -pix_fmt yuv420p -color_range mpeg -r 15 -f rawvideo -preset ultrafast -tune zerolatency -filter:v scale='min(1280,iw)':'min(720,ih)':force_original_aspect_ratio=decrease,scale=trunc(iw/2)*2:trunc(ih/2)*2 -b:v 299k -payload_type 99 -ssrc 2133868 -f rtp -srtp_out_suite AES_CM_128_HMAC_SHA1_80 -srtp_out_params F/G9NqqhQ6wtcGpD12ffTEGhGs18X6983HdmmhsG srtp://192.168.178.51:49600?rtcpport=49600&pkt_size=188 -loglevel level+verbose
```

**Environment:**

* **Node.js Version**: v12.19.0
* **NPM Version**: 6.14.8
* **Homebridge Version**: 1.1.6
* **Homebridge Camera FFmpeg Version**: 3.0.3
* **Homebridge Config UI X Plugin Version**: 4.31.0
* **Operating System**: Raspbian


