Add support for custom icons.

Created by: natansh

Ideally, a custom icon should be as easy to add as

.icon-[custom name] {
    background-image: ...;
    background-position: ...;
}

Apparently, this used to work in 2.0.4, but no longer works in 2.1.0 - someone reported this in the mailing group. The issue is that the bootstrap styles in many elements have more specificity now, such as

.navbar-inverse .nav > .active > a > [class^="icon-"]

which makes the browser pick that particular background image (mentioned in the property).

I have been thinking of some solutions and the best I could think of was to have a separate class prefix, like

.custom-icon-[custom name]

I have added support for using this approach in the commit.

Would love to hear/know about any alternate approaches possible.