Fix indent overflow

Fixes #4

The main issue here is that `document.execCommand("indent")` nests the selected line in blockquotes. Eventually, the summed margins of these nested blockquotes will cause the paragraph to overflow out of the editor div. I've fixed this by comparing the sum of blockquote margins to the width of the editor, so this is flexible if you later decide to change the default indent size.

I also moved the scripts to the bottom of `index.html` to ensure that DOM manipulation (needed for computing blockquote margin) functions correctly. It's good style anyway to put JavaScript at the end of the body, since it blocks rendering. :)