Add Minitest support to TagProf

[lioneldebauge]

What is the purpose of this pull request?

Following this discussion this PR aims to add Minitest support for TagProf.

What changes did you make? (overview)

Created a TagProfReporter class which reproduces TestProf::TagProf::RSpecListener behavior. The main differences are:

• because tags do not exist in Minitest we use subdirectories of main test directory to group tests statistics
• when a main test directory cannot be determined it groups by test and display a significant warning message

Is there anything you'd like reviewers to focus on?

You can read the comments for my different questions :)

Checklist

• [x] I've added tests for this change
• [x] I've added a Changelog entry
• [x] I've updated a documentation