Add a checkbox for CodeMirror "smartIndent" option.

Depending on situation, CodeMirror's default "smart indent" option may have it's own views on what indentation. Sometimes, questionable views:
![Example](http://i.imgur.com/lwa5dXh.gif)
In larger userstyles, the thing may glitch out completely, to the point where it changes the indentation level midst the block. Workflow becomes slightly less enjoyable when you have to constantly fight the code editor, as you can guess.
This pull request adds a checkbox for that option. With the option turned off, CodeMirror will only indent new lines to previous indentation level, similar to how most code editors do by default. I did not miss any required parts of integration, as far as I'm concerned.