[feature] Support addition of results

[samueldg]

It would be nice to be able to aggregate the reading time of multiple texts.

a = readtime.of_text('hello')
b = readtime.of_text('world')
c = a + b

Which currently errors out with:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'Result' and 'Result'

[alanhamlett]

Added in v1.0.4.