Implementation of a basic test framework for IEx. Closes #919
Write tests for IEx helpers
Rewrite tests for helpers to use capture_iex and capture_io sparingly
Update the tests for l and r helpers
Don't process ANSI escapes in ExUnit output
Add more tests for shell coloring
Add docs for IEx test framework
Clean up test code for module helpers
Update the r helper
Update the docs for Code.load_file and Code.require_file
Add prompt test
Proof-read the doc for capture_io
Minor comment cleanup
Merge pull request #1137 from alco/919-iex-test-fmwk

Test framework for IEx