Allow logo option to treat any value as src (external, base64, relative, etc.)

[jhildenbiddle]

Docsify assumes the logo option value is an absolute path. This prevents the use of several valid image src values.

Example:

logo: "logo.svg"

Output:

<img alt="docsify-themeable" src="/logo.svg">

Notice the auto-inserted forward slash at the beginning of the src attribute. This behavior prevents valid image sources from being used:

Base64:

logo: "data:image/png;base64, ..."

<img alt="docsify-themeable" src="/data:image/png;base64, ...">

Relative path:

logo: "../logo.svg"

<img alt="docsify-themeable" src="/../logo.svg">

External URL:

logo: "https://docsify.js.org/_media/icon.svg"

<img alt="docsify-themeable" src="/https://docsify.js.org/_media/icon.svg">

The fix seems straightforward: just use the logo option value as-is for the image src.

Thanks!