feature: option to import other files for headerTemplate

[Ookamini95]

Problem:
I'm using headerTemplate to generate a custom header for each page of my file, but since it contains an image and I'm using the cli I have a large blob of text from the data:image/png;base64 (using code this would be trivial to solve, just a couple of function calls and replacements, but that would go against the point of directly converting md to pdf).

Solution:

Simple solution would be providing the path of the headerTemplate/footerTemplate.html

[Artagos]

getting the same problem, if only we could see the headers html path