path: added parse() and format() functions

The parse() function splits a path and returns an object
with the different elements. The format() function is the
reverse of this and adds an objects corresponding path
elements to make up a string. Fixes #6976.

Fixes: https://github.com/joyent/node/issues/6976
PR-URL: https://github.com/joyent/node/pull/8750


Reviewed-by: Julien Gilli <julien.gilli@joyent.com>