Add uid and gid options for user impersonation

Hello again @chjj,

Here's another pull request allowing to run the pty as another user. It fixes @filirom1's #23.
The added test is disabled when not root, I haven't spent too much time looking for a way to sudo this test.

Is anyone familiar with node addons aware of better ways to provide parameters ? I found it not so natural to add those 2 optional parameters.