Allow for passing custom messages

[diedexx]

WIP: Still need to update tests

Context

• The built-in messaging is very specific for PHP 5.6 which is no longer relevant. Instead of having to release WHIP with a new message for every PHP version, we need to be able to provide a custom message.

Summary

This PR can be summarized in the following changelog entry:

• Add support for customized messages.

Relevant technical choices:

• BC break: the WordPress fasade now requires an array per component instead of just a version constraint. This is done to match a version requirement to message.
• BC break: I've added version and operator to the WHIP_Requirement interface. The code already relied on these functions. and in the context of WHIP every requirement is a version constraint, so I didn't think adding w WHIL_Version_Requirement. was the way to go.

[diedexx]

No longer relevant - closing