Merge pull request #1128 from alco/1020-extended-doctest

Implement extended doctests
Make IEx.boot_config public
Initial implementation of capture_io-based tests for IEx
Only start :iex for interactive tests
Keep IEx options in the test context
Add an example of using context in teardown
Add itest and make it look more like a framework
Set IEx.Options outside of capture_io context
Make itest macro smarter
Add support for arbitrary regexes in itest
Support for testing .iex file loading