Different download options for external core sites

[simonbaird]

I can think of three ways to download an "external core" site:

• Download as an external core file with the core url set to https://tiddlyhost.com/tiddlywikicore (Actually this is what you get right now by default if you download.) - The TiddlyWiki will work from a local file provided you have internet access and Tiddlyhost (or whatever CDN it is hosted on) is up.
• Download with the core url set to src="tiddlywikicore.$ver.js" and provide an option to download the core file as well with some instructions to explain that it's needed. - The TiddlyWiki will work from a local file provided you have the core js file sitting alongside in the same directory.
• Convert the external core to a self contained TiddlyWiki and download it. - Best for offline use since there's nothing else needed.

This last one suggests the idea of being able to convert between self-contained and internal-core easily. I like that idea, but let's say that's out of scope for this issue.

[simonbaird]

The first two options are available now. The third option might happen some day via #287 .