Image saving path should support interpolation.

[allenlsy]

For example, the image saving path should support to be set as {{filename}}.media, and {{filename}} is extracted from the current file name.

This is a typical use case. If I have many md files in one folder, and I want to store their images in separate media folders.

[kiyoka]

I am also using Unotes for a similar use case. I'll try to figure out what way to specify.