Option to turn off links syntax

Hi,

we're using marked for a phonegap/cordova app and links in that context is not a good thing. I've added a 'links' option that just turns links parsing off. Also I refactored setOptions a bit so that it "inherits" default options. 

I haven't had time to to run or add tests for this and I understand if you don't want to burden the codebase with yet another option, so take it or leave it as you please :-)

cheers!