Adds support for stackrel using a subclass of MTLargeOperator.

[cupojoe]

Closes #63

[cupojoe]

@kostub anything on this feature?

[cupojoe]

@kostub any updates?

[kostub]

So stackrel is a macro - not one of the basic TeX types. should it be implemented as a macro? otherwise the number of native commands will increase significantly.

Also the following question says stackrel is deprecated and we should use underset instead.
https://tex.stackexchange.com/questions/39225/differences-between-stackrel-and-stackbin

[cupojoe]

That is an interesting point. Is underset also a macro? How do you propose we implement macros? I'd like to provide support for stacking one way or another.

_{Sent with GitHawk}

[cupojoe]

@kostub do you have any more insight as to how you would like to see this implemented? I'm currently using my fork with the stack rel implementation, but I'd like to get these changes into your project and go back to using the supported pod.