Support for files in branch other than main or master

[benclmnt]

Use case: It is nice to be able to visualise some files in a development branch

[Wattenberger]

this would be a great future! In case it's useful now, you can view any file by specifying the SHA of a commit in the url:

https://flatgithub.com/org/repo?filename=data.csv&sha=[[ SHA OF COMMIT ]]