Feature: custom filters

[metheoryt]

Hi, and many thanks for such an awesome project!
Currently I'm seeking for a solution to add a custom watermark to an image in Imagor, with a specific color blend logic, and none of variants (watermark, label) fit.
And I wonder, if there's a way to implement and add a custom filter to Imagor, maybe in thumbor-style?
Thanks in advance!

[github-actions]

This issue is stale because it has been open 30 days with no activity. Remove the stale label or add a comment, or this issue will be closed in 15 days. You can always re-open if you still feel this is still an issue. Tag @cshum for more information.

[github-actions]

This issue was closed because it has been stale for 15 days with no activity.