Update composer deps + readme.
Minor readme changes [ci skip]
Housekeeping.
Header depth fix [ci skip]
Minor typo
Change abstract comparator to only test comparability once in each direction, updated to archer 1.0.0-alpha.1
Updated readme badges [ci skip]
Housekeeping.
Re-worked comparable interfaces such that an behavior must be specified explicitly.

* Added SelfComparableInterface (fixes #7)
* Separated comparators such that they can be composed, rather than inherited (fixes #9).
* Removed $deep parameter from Parity::compare()
* Also fixed #7
Readme improvements [ci skip]
Readme improvements [ci skip]
Update README.md [ci skip]
Moved example above concepts [ci skip]
Added use statement in readme example [ci skip]
Typo [ci skip]
Added ComparatorInterface::__invoke() - fixes #6
README.md update [ci skip]
README.md update [ci skip]
README.md update [ci skip]
Bumped version and updated changelog.