Enhancement Request: 80 Column ruler

[modcap]

It is a pretty standard convention to line-break ascii docs at column 80, and it would be terrific if there was a checkbox in the options that can enable a simple line at column 80.

I have been using inspector to add the following class every time I use the editor:

.CodeMirror-code {
    max-width: 82ch; /* for some reason I haven't researched setting to 80ch only allows 78 chars */
    border-right: 1px solid #eee;
}

I'm not sure if .CodeMirror-code would be the best choice for where to add this (a transparent absolutely positioned div may be better) but I think this would help a lot of readme/wiki editors.

Thanks for the great extension either way.