Add left aligned form feedback icon

It's a relatively common use case to add a glyphicon *inside* of an input box on the left and right.  So much of that functionality has been developed with `form-control-feedback` that it would be nice to make a few minor tweaks to add the ability to left align the icon as well.

#### [Here is a demo in plunker illustrating most of the possible alignment scenarios][plunker]

I used this as a test case to make sure that the alignment stayed true across multiple form configurations, including basic, horizontal, inline, alternative sizing, with and without labels, and using both glyphicons and font awesome.

To get a scope of the desire to do this natively with Bootstrap, here a [stack overflow question I answered about adding a Glyphicon to an Input Box][so] 

[so]: http://stackoverflow.com/a/18839305/1366033
[plunker]: http://plnkr.co/edit/ZrnnkFR3Yv9HGQHrRMiW?p=preview
When the padding mirrors the input height, it makes the icon look like it occupies a square on the side of the control.  This looks much better on the range of control sizes, including small, default, and large.  This difference is much more pronounced when the icon comes first on the left because the padding plays a much bigger role.