Excluded namespace doesn't work with '*'

[esetnik]

in the visionmedia/debug command line you can specify an exclusion like:

DEBUG=*,-something

But this doesn't work in this package. I'm not sure if the goal is to be compatible with debug, but this causes issues such as https://github.com/winstonjs/winston/issues/1715 with users unable to turn off the debug messages coming from diagnostics package.

I've added a failing test case in #21