Define feature: "Injecting a page object directly into the context".
Create PageObjectArgumentResolver.

Unfortunately it will not work with the way contexts are currently initialised, as mink session is prepared on before scenario or example events, and context is initialised on before suite. Preparing default mink session on before suite event fixes the problem, but in some cases page object would get a wrong session.
Inject page object proxies to postpone initialisation.

This helps to avoid the problem with default session not being available at the time page object is injected to a context file.
Add type hints.
Improve readability.
Make proxies optional.
Document injecting page objects to contexts.
Simplify the factory definition processing.
Fix a typo.
Merge pull request #33 from jakzal/page-object-argument-resolver

Page object argument resolver