Add support for themes in mermaid

[Wiskerke]

Hi,

This merge request mainly adds support for configuring themes, with improved support for darkmode. (I mainly use darkmode and the default experience with the silverbullet-mermaid plug is not great. And with these changes you can at least configure it with better alternatives.)

I've updated the readme.md (and the plug has the same copy pasted content) with the explanation of how the theming works.

In addition there are a couple of small refactorings / fixes:

• Since config cannot be undefined (we are providing a default value to it), don't bother with the config?..
• Moved the version default outside the getConfig, to make it more consistent with the rest of the code.
• Switch from the deprecated 'mermaid.init' method to 'run'
• The 'click' event is now doing an alternative action to refocus the editor, since 'api' is no longer working.

[Wiskerke]

I am wondering a bit if I'm not making it too big here. I like the whole package, but it does make this plug less suitable as an example plug.

@zefhemel : If you like I could create a pull request that only supports the 'theme' and 'theme_darkmode' from the settings. I think that is sort of the minimum to make it useful for darkmode.

I do like the fillBackground option, but that is the main feature that adds the complexity. As fillBackground is something that makes sense only to enable on some diagrams (and then you'd need frontmatter parsing), and then it is really nice to be able to configure the background color...