Keep a record of source position in output of lexer

[addrummond]

It would be useful if the objects in the array returned by the lexer contained start and end indices so that the markdown source corresponding to each object could be recovered. I'd like to use marked to parse markdown documents and pull out individual sections. It would be nice to be able to store these sections as markdown source, since the object format returned by the lexer could change in future. I'm planning to have a go at adding this feature myself, but I wondered if people think that it's an appropriate addition.

[scottgonzalez]

Duplicate of #441.