Added flip, rotation & ratio support

[rpeyron]

This PR adds support for options flipH, flipV, rotate90 and streamRatio available for default OctoPrint webcam to MultiCam. It is now possible to have different values for each webcam. When migrating parameters, current webcam values will be taken for the newly added parameters.

Also extra little changes of how the plugin works:

• it will internally change directly the settings of the webcam to the selected one. I made this change to fix the annoying bug when changing tab
• fixed a bug with saving parameters not working after multiple times
• the snapshot URL is also handled, but not visible in the settings screen because of a lack of UI space :) (and I am not sure of the use of it)

[mikedmor]

Thanks for the updates, i have merged your changes into the repo. Let me know if you would like me to add a credit to you into the readme.

[rpeyron]

Thanks for the merge. No need to worry about creating a contributor section only for me, maybe when there will be lot more contributors :) Thanks for this so useful plugin.