options.prettyPrint can now be a function

If options.prettyPrint is a function, meta will be run through it before appending to output.
options.prettyPrint can now be a function

If options.prettyPrint is a function, meta will be run through it before appending to output.
Merge branch 'master' of github.com:zuk/winston

Conflicts:
	lib/winston/common.js
Merge branch 'master' of github.com:flatiron/winston
Merge pull request #274 from zuk/master

options.prettyPrint can now be a function for custom pretty printing