intuitive accordions in settings > extensions

[Daniel-Hug]

After reading sualeh's comment on #137, I realized that I had had the same experience when I first started using StackEdit (not knowing that each extension in settings > extensions was an accordion dropdown with more options):

I don't see any place to paste code into Settings->Extension->User Custom. It is just a checkbox. In any case, I think adding in custom CSS should be made even easier than that, for users that are not very technical.

This commit is an attempt to make the accordions more intuitive.

[benweet]

I will do something like this in the next major version. Thanks for the idea.

[benweet]

I've done something similar in StackEdit 4 beta. See https://stackedit-beta.herokuapp.com/.