btn-xxx-inverse class for hight contrasted website

I have edit buttons.less to add inverse feature on button (like navbar).

In hight contrasted WebSite (navbar, header) standards btn-xxx styles can be useless in site header or body.

btn-outline getbootstrap.com homepage inspire this feature.

I use the variable @body-bg but I think it's not the best way (a transparent variable will better but need to add new variable).

### Example or Test
* Options : http://jsfiddle.net/hQv22/
* Sizes : http://jsfiddle.net/A5V3R/1/
* Active state : http://jsfiddle.net/mvrq4/2/
* Disabled state : http://jsfiddle.net/mvrq4/3/
* Button tags : http://jsfiddle.net/mvrq4/4/
* Glyphicon : http://jsfiddle.net/mvrq4/5/
* DropDown : http://jsfiddle.net/mvrq4/6/
* Button groups : http://jsfiddle.net/mvrq4/7/
* Button toolbar : http://jsfiddle.net/mvrq4/8/
* Button toolbar Sizing : http://jsfiddle.net/mvrq4/9/
* Button toolbar Nesting : http://jsfiddle.net/mvrq4/10/
* Button toolbar Vertical variation : http://jsfiddle.net/mvrq4/11/
* Justified button groups : http://jsfiddle.net/mvrq4/12/
* Split button dropdowns : http://jsfiddle.net/mvrq4/13/
* Dropup variation : http://jsfiddle.net/mvrq4/14/
* Button addons : http://jsfiddle.net/mvrq4/15/
* Inverted navbar : http://jsfiddle.net/mvrq4/16/
* With badge : http://jsfiddle.net/mvrq4/17/