feat: add parameter flip to the client endpoint

[zihaooo]

There are four orientations in total for the reMarkable 2 screen: two for portrait and two for landscape.
When the orientation of the reMarkable 2 screen changes, I also want goMarkableStream to rotate.
Since there is already an endpoint parameter portrait to control the portrait or landscape, I add another parameter flip to control if it needs to rotate 180 degrees at each case.

[owulveryck]

Looks good to me and this fixes #105

Thank you very much for your contribution