Fix chaining to work and still print True/False

[charlesdunbar]

Description

Currently, chaining does not work per the README as the match returns a boolean instead of an
expect. This fixes it to still work with multiple expects, while still allowing for True/False to be printed via an interactive prompt

Examples

Current code, on a prompt:

>>> from robber import expect
>>> expect(1 + 1).to.be.an.integer()
True
>>> expect(1 + 1).to.be.an.integer().within(1,3)
AttributeError: 'bool' object has no attribute 'within'
>>> expect(1 + 1).to.be.an.integer().within(5,10)
AttributeError: 'bool' object has no attribute 'within'

With this PR:

>>> from robber import expect
>>> expect(1 + 1).to.be.an.integer()
True
>>> expect(1 + 1).to.be.an.integer().within(1,3)
True
>>> expect(1 + 1).to.be.an.integer().within(5,10)
robber.bad_expectation.BadExpectation:
A = 2
B = 5
C = 10
Expected A to be within B and C