Add markdown field support through SimpleMDE

[d9su]

I was looking for markdown editing support in rails_admin and came across this stale issue #1235. After some digging I couldn't find a satisfactory solution in built-in support or plugins, so thought I'd take a stab at it myself.

I decided to go with SimpleMDE after some research since it looks neat and very simple to use.

With this feature, you can do field :text_field, :simple_mde and have a SimpleMDE markdown editor. The easiest way to enable it is simplemde-rails which unfortunately uses a somewhat outdated SimpleMDE version, but I've opened an issue there to hopefully update it fixed in KHN190/simplemde-rails#1. Users can also just load SimpleMDE themselves.

This is how it looks like:

Test coverage:

Linter is complaining that 2 lines in the coffeescripts are too long... but there are longer lines existing in the same file, not sure if I should bother with it.

cc @mshibuya

[d9su]

Hey hey, may I get some thoughts on this? It'd be great if we can get it in the gem, so let me know what I can do to achieve that 😃

[coorasse]

Would be really glad to see this feature 👍

[philschoefer]

This would be a great addition. Please merge!

[mshibuya]

Nice work, thanks!