feat: custom css option.

[y2w8]

Add custom CSS support

Moves the plugin's default styles out of index.html into a dedicated assets/style.css file.
On start, the stylesheet is inlined into the generated index.html via a <style> tag.
A new custom_css config option lets users point to their own stylesheet:

require("markdown_preview").setup({
  -- Leave it empty for default style only
  custom_css = "~/my-styles.css",
})