Add config `Expression`s (#199)
* Add snippet from Apache CollectionUtils
* Expression parsing: initial commit
* Don't recursively create new readers. Private method that accepts reader instead.
* fixup access modifiers, minor readability
* convert the Actions into methods on `ParsingContext`
* match LogicalOperator w/ switch instead of if
* Remove ExpressionOperand, replace with base Expression. Rename PatternMatchingExpression to BinaryExpression.
* Fix parsing bugs. Add basic expression evaluator (Expression.matches(), Collection evaluation context)
