"Base URL" for resolving inline image relative references

I've got a bunch of Wiki pages on GitLab which include images. Images on their system are referenced using paths **_relative to the wiki root_**, without any protocol/host name. E.g.:

```md
```
This is fine, and works great. (I can paste the images right into the editor!) BUT, when I check out the wiki git repo and load it into _Macdown_ I get a **_broken image_**.


To fix it, I have to edit the image reference to include the host path before the image path:

```md
```

Here's my request: **_Is it possible to have a document-specific Base URL for images?_**

I'm not sure how this would best be implemented, but I'm thinking maybe some hidden pragma in the document? Such as:

```md
[macdown_base_image_url]: # (https://foo.bar.bat/path/to/base/)
```
FYI, The idea for that syntax comes from:

https://stackoverflow.com/questions/4823468/comments-in-markdown

Thoughts? I realize this may open a can of worms (setting variables in a MD document).

No files, tests, or entry points are named in the issue. Start by locating MacDown's Markdown image-resolution path and reviewing how document-level metadata is handled; done means a document-specific base URL resolves relative image references without breaking existing Markdown behavior.

Written by the indexing model from the issue text.