feat: allow custom date format

[thenbe]

First, thanks for this tidy utility.

What

This change allows users to set a custom date format using an environment variable ROFI_SCREENSHOT_DATE_FORMAT.

Why?

1. The current hardcoded format contains a space. File names without spaces are simpler to deal with in the terminal.
2. This would allow setting more robust formats. For example, using a date format like +%Y-%m-%d-%H-%M-%S will cause the files to always have sane sorting, even when viewed through file trees that may not explicitly support sorting by date (eg. the file tree in your editor).