chore(readme): add `with_mock` documentation

Previously the README documentation only provided an example of the
`test_with_mock` macro. No examples were provided for the `with_mock`
macro. An example of the `with_mock` macro has now been added to the
README.
feat(test_with_mock): receive `context` as an argument

A new `test_with_mock/6` function has been added to the Mock module.
This new function differs from the `test_with_mock/5` function by
accepting a context argument.

This allows `test_with_mock` to be used with the standard ExUnit setup
procedure, enabling information to be shared between callbacks and
tests.
Merge pull request #18 from mattfreer/feat/with_context

Feat/with context