Add test for greater/less than (#624)

Add test for greater/less than (#624)

It can be helpful to have a test for values that are greater or less than
another value, especially when looping over lists and using `select()` or
`selectattr()` filters. This patch adds tests that allow a developer to
check if a value is greater or less than another value.

For example, if I have a list of vehicles and I want to find the motorcycles:

    vehicles | selectattr('wheels', 'lessthan', 3)

Or, if I want to find large trucks:

    vehicles | selectattr('wheels', 'greaterthan', 4)

Tests are included.