Custom evaluate

[stefnotch]

#31 (comment)

I took a look at this and there are two quite reasonable options:

1. Adding \\solve, \\factor, and \\expand to the dictionary like you suggested. If we do that, then we should also make it possible to call them like \\solve(3x+5==5, x)
2. Adding the ability to parse unknown functions and using that Unknown function parsing #33 However, for that we'll need to wait for the next compute engine version