add config note to readme
move nock recorder options into a recorder property in the options
add a defaultConfig that can be overridden and move excludeScope to there
bump mocha version
everything through config rather than globals
pass describeFixture options as secondOption

This keeps the config closer to the describeFixture definition which makes it
easier to find. It also makes it so that if you pass it to just describe on
accident it won't work so it prevents a hard to debug error from happening.