Add ability to watermark images by parameter

[fluffy-critter]

Expected Behavior

For purposes of protecting images there should be a watermark parameter which lets you add an arbitrary text string to the image as well as the file name and EXIF. This should also propagate as a full screen attribute.

Example: ![{watermark="beesbuzz.biz for "~user.name}](sexypic.jpg)

Current Behavior

Possible Solution

Steps to Reproduce (for bugs)

Context

[fluffy-critter]

Note that the above Markdown won't work because Markdown doesn't execute in a Jinja context, so the ~ operator and Jinja context variables aren't available. Maybe there should be some additional templating stuff available to the Markdown engine, or something.

[fluffy-critter]

It's possible that Python f""-strings are available from the Jinja context. Or simply always provide a .format(user=user,request=request,...) to the string so that the invocation could be something like:

![{watermark="beesbuzz.biz for {user.name}"}](image.jpg)

although that in turn might cause some weird parse issues for the Markdown extension.