Fixed some cycling in the math completions.

In the math completions, there were cycles in tab completions (like cd
-> \\cdots -> \\cdots -> \\cdot -> . . .). When writing LaTeX, they did
did not function as I suspected, so I modified them so they cycle
through the commands.
Added the ability to replace current environment with another

By pressing C-l, u, the user can specify what to change their current
environment to.