Allow custom matcher definition

[terrainoob]

Allow users to add custom matchers/matcher libraries (like https://arjanvandergaag.nl/blog/rspec-matchers.html)

This will allow us to add things like:

• specific DR/game dev matchers
• array matchers
• xml matchers
• json matchers etc.

[levaleureux]

@terrainoob this PR do 2 things https://github.com/levaleureux/dr_spec/pull/9/files So I will extract matcher from it and create a PR for develop branch.