Added back exit code support

[twolfson]

In #88 and #89, we moved to PROMPT_COMMAND and added exit code support. In #90, we discovered we broke existing PS1 extension mechanisms due to always setting the variable

This PR moves to a hybrid where we track the items we need for exit code support via a PROMPT_COMMAND hook and exposes them as globals

In this PR:

• Added back same exit code config, logic, and documentation
• Added back Travis CI notification patch
• Patched PROMPT_COMMAND as a hook, to only expose globals for PS1
• Added PROMPT_COMMAND extension logic
• Updated PS1 to use globals from PROMPT_COMMAND
• Updated tests
• Fixes Exit code reporting #82 again

/cc @rpdelaney

[twolfson]

Also, fwiw -- we did sanity check that this works with the PS1 extension as reported in #90 again =)

[twolfson]

Guessing @rpdelaney is still in the process of moving. It's been 11 days since I wrote this though so I've forgotten everything. Going to give it a review and likely land it

[twolfson]

LGTM

[twolfson]

This has been landed in 1.1.0