Add _.fetch method

[mottalrd]

Hi,

does underscorejs has something like fetch in ruby (http://ruby-doc.org/core-2.1.5/Hash.html#method-i-fetch) ?

I implemented my own extension to do that but I was wondering if you are interested in adding it to underscorejs. https://github.com/mottalrd/underscore-fetch/blob/master/lib/underscore-fetch.js

Thank you

[mottalrd]

Any feedback on this?