Default theme option

[Bassetts]

Opening a PR to initiate discussion.

Everything is there and working, but there are a few things I dislike/am unsure of.

1. Calling this option default-theme is confusing to the user. The options are presented as 'themes' to the visitor in the theme popup, but from a user's perspective the theme is the whole theme inside src\theme. Maybe this should be default-style or default-theme-style? Maybe I just need to explain the option better in the documentation?
2. I created a new handlebars helper to append (default) to the selected default theme in the theme popup. Originally I had done this in the javascript that sets the theme on page load, but it seemed a bit hacky when we know which theme is the default during render. Not sure if this is the best way, or best implementation of a helper though.
3. I have left the ordering of the theme popup as it is, but this means the default may now not be the top item. Should we re-order the list of themes to always have the default as the first item?

Once complete this will resolve #95 and #713

[Michael-F-Bryan]

Calling this option default-theme is confusing to the user

I think default-theme should be okay. People will probably be more familiar with the drop-down than the theme/ directory, and someone playing around with theme/ should understand the difference. Maybe we could update the documentation to mention this only affects the colour schemes selected by default from the themes drop-down, and is totally separate from the theme/ directory?

[Bassetts]

@Michael-F-Bryan realised I forgot to comment. I have updated the documentation to specify this affects the themes drop-down.

[Michael-F-Bryan]

Merged 🎉 Thanks for all the hard work @Bassetts!

[fitzgen]

Awesome! 🎉