Feature Request: Env Variable for Timestamp Format

[arichiardi]

Hi there, just discovered this awesome tool and I think the frustration is shared (especially cause byzanz is not present in the Arch repos anymore!).

One thing I wanted to kindly ask is a way to control the timestamp from the environment (same you added for the ROFI_SCREENSHOT_DIR.

A ROFI_SCREENSHOT_TIMESTAMP would solve the issue of not being able to pass the parameter at the command line when using rofi.

I might be able to take a look at this if you like the idea and thanks in advance!