Enable raw packet manipulation as library code

[Aptimex]

I maintain a project that uses wireguard-go as library code for userspace Wireguard functionality. My project had a need to modify some options and functionality of the low-level networking functionality that this library offers, but which is currently not exposed when used as a library import. Since that flexibility might be useful to other projects as well I'm submitting this PR for consideration.

This PR provides 3 changes:

• Add a Stack() method that provides access to the private net.stack object
• Add a Endpoint() method that provides access to the private net.endpoint object
• Add a new CreateNetTUNwithOptions() function that is exactly the same as the CreateNetTUN() function except it allows arbitrary stack options to be passed in; CreateNetTUN() then calls the new function with the current default options, which avoids breaking any existing code.

Edit ~2 hours after initial PR: figured out a much cleaner way to expose stack options, updated this description to match the new commit.

[Aptimex]

Closing due to messy commit history and failed DCO checks, moved to #127