Fix pretty print

[WilliamRagstad]

Now when some expressions and atoms are being printed, they do not adapt to the previous indentation.
Fix this by adding a parameter to toString on both Expr and Atom which holds a "previous indentation" string and indent all lines with this concatenated to at the beginning of the string.
There should also be a setting somewhere (maybe on the Interpreter) specifying indentation type (2 spaces/ 4 spaces/tabs/etc).