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
If using font awesome, the library is likely to come after bootstrap and override `line-height` set here.  I'm inclined to use `!important` to make it easier for these line heights to persist. If the icons are being used explicitly within an input box, it should always be the case that their height should match that input box so marking them as important shouldn't cause too much confusion.