Delete model stuff

The model library has been moved to the mlk gem:
https://github.com/pfleidi/mlk
Remove mlk tests; move test/spec -> test/unit
Rename test -> spec
Rename test tasks; add combined test task; fix Mlk deps
Replace Model with extracted Mlk::Model
Require file storage
Use minitest/pride for colorful test output
Refactor text filtering pipeline and tests
Add rendering helper and tests
Remove unneeded text content test