Support for stencil-only formats

[GiantBlargg]

As per of the OpenGL 4.4 spec:
STENCIL_INDEX8 can be used as the internal format for Renderbuffers and Textures
STENCIL_ATTACHMENT is a Framebuffer attachment

But neither of these are present in their respective enums.
(I am aware that these values can be used by casting the appropriate int value)

Stencil-only formats are not often used so there may be other overlooked issues regarding this.