Identify binary operators used as unary operators

[sigmavirus24]

Previously we handled the case where binary operators were being used
as unary operators except in the case where they followed another
binary operator, e.g.,

foo = (1 +
       -10 *
       2 /
       -5)

This change updates the check for W503 to track the previous
non-whitespace token type and token text to check if it is in fact also
a binary operator (because you should never have two binary operators in
a row).

This does not handle invalid syntax, e.g.,

foo = (bar /
       /baz)

But the false-positive generated for other cases was more harmful than
not catching what will instead be caught by the interpreter.

Closes gh-484

[rkern]

Works for me! Thanks!

[IanLee1521]

Thanks @sigmavirus24 + @rkern !