Make it easy to add your own symbol aliases

[holyjak]

Symbol aliases make it possible to translate operators used in symbolic Clojure expressions passed to Wolframite to the corresponding Wolfram functions and vice versa. See runtime.defaults/base-aliases for what is supported out of the box.

We should make it easy to add custom aliases, such as ** <-> Power (though this one & friends should perhaps rather become a part of the default set).

Related to / need for #19

Proposed by @light-matters