Added a mixin for keyframes that removes redundant prefixes in the keyframe's content

After looking at a few comments on a keyframe mixin (https://github.com/thoughtbot/bourbon/issues/98), and finding at a few solutions for Compass (https://github.com/ZeeAgency/compass-animation/blob/master/sass/_keyframes.sass)... I've added enable/disable flags to the prefixer, which allows for keyframes to be included, without redundant prefixes in its content.

For example, the following code:
```
@include keyframes(megatrippy) {
	from {
		background-position: top left;
		@include transform(rotate(0deg));
	}
	to {
		background-position: top right;
		@include transform(rotate(360deg));
	}
}
```

Produces the following CSS:

```
@-moz-keyframes megatrippy {
  from {
    background-position: top left;
    -moz-transform: rotate(0deg);
  }
  to {
    background-position: top right;
    -moz-transform: rotate(360deg);
  }
}

@-webkit-keyframes megatrippy {
  from {
    background-position: top left;
    -webkit-transform: rotate(0deg);
  }
  to {
    background-position: top right;
    -webkit-transform: rotate(360deg);
  }
}

@-o-keyframes megatrippy {
  from {
    background-position: top left;
    -o-transform: rotate(0deg);
  }
  to {
    background-position: top right;
    -o-transform: rotate(360deg);
  }
}

@-ms-keyframes megatrippy {
  from {
    background-position: top left;
    -ms-transform: rotate(0deg);
  }
  to {
    background-position: top right;
    -ms-transform: rotate(360deg);
  }
}

@keyframes megatrippy {
  from {
    background-position: top left;
    transform: rotate(0deg);
  }
  to {
    background-position: top right;
    transform: rotate(360deg);
  }
}
```