feature request: add url to external css

[se4203]

Issue description

Issue: The textbox for custom css truncates long inputs.
Instead of making it accept long inputs, I suggest adding an url.
The exported html code will then contain something like <link rel="stylesheet" href="https://example.com/resources/css/custom_md.css"> instead of the normal <style>...</style> element.

Environment

• Notepad++ 8.14 (32bit)_
• MarkdownViewer++ 0.8.2.25525
• Windows 10 (64bit)

[hanksmoody]

Support for referenced css files would also be a nice feature in this context. For example:

@import url("referenced.css"); .header-placeholder { height: 1px; width: 100%; margin-top: 160px; }