Allow option of `to` for separator (in addition to >).

Have you considering allowing the word 'to' for the separator? Obviously it's not localized but it's only a very specific case here and optional.

The following reads better and makes more sense without a mathematical operator as a separator:

`u 1km to mile` > `u 1km > mile`

(Note that I don't code in Python so this code is from a few Stack Overflow queries. Perhaps you'd consider refactoring this to allow for an array input or something.)