Add enableDataChannel option to peer

[marcelwgn]

What is the purpose of this pull request? (put an "X" next to item)

[ ] Documentation update [ ] Bug fix [x] New feature [ ] Other, please explain:

What changes did you make? (Give an overview)

Added enableDataChannel option to simple-peer which allows to omit data channel creation for a peer.

This PR also adds necessary tests to ensure that the flag is being respected properly.

Which issue (if any) does this pull request address?

Closes #360, Closes #792

Is there anything you'd like reviewers to focus on?

[marcelwgn]

Don't want to be pushy but is there anything blocking this PR @feross ?

[cedrouck]

@feross, it would be very helpful if we can merge it. Do you think we can plan it soonly ? Thanks