intuitive accordions in settings > extensions

After reading [sualeh's comment](https://github.com/benweet/stackedit/issues/137#issuecomment-26301268) 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.