Time stamp has invalid characters in Windows

[ScribbleGhost]

In a Windows environment, the following characters cannot be used in a file or folder name:

< (less than) > (greater than) : (colon) " (double quote) / (forward slash) \ (backslash) | (vertical bar or pipe) ? (question mark) * (asterisk) Microsoft Docs

So the colons in the date/time stamp should be changed to something more OS-agnostic. Perhaps hyphens. https://github.com/Kagami/mpv_slicing/blob/d09c11227704c8d5bdaa2c799ef64dce881c63a7/slicing.lua#L30

If these characters are not changed the FFmpeg command will not work in Windows.