[Feature] Support to customize image src

[yfwz100]

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Why you want this feature?
When embedding muya in native application, we need to redirect the relative path to another local path.

Describe the feature request
When embedding muya in native application, the editor is loaded in a custom or file-based scheme and the markdown content is not loaded from the same domain. We need to redirect the relative path to the a preset base path.

Additional context
Add any other info about the feature request here.