Merge pull request #10 from klauspost/easy-check-for-past-shutdown

Return a nil notifier if shutdown already started

Merge pull request #9 from muesli/master

Fixed typo in README.md

Use exclusive lock

Lock logger when writing.

Add optional custom context to functions. (#5)

This will allow passing custom information on individual notifiers to tell them apart.

* Adds custom prefix to warnings.
* Add lock to printer for convenience.
* Drop Go 1.5 CI tests :(

golang/lint#246