Rename pact-node

[mefellows]

See #145 and other conversations.

Candidate names:

• pact-cli

Reasons not to do this:

• Impact to the ecosystem (as at today, there are 1.1k known uses on Github - this could break a a lot of things)
• We're deprecating it over time in favour of Rust, so why go through this pain
• Renaming in may not actually help the confusion

[TimothyJones]

We've renamed pact-node to pact-core (and the repo to pact-js-core) for the upcoming new release of pact-js (currently in beta). This will break pact-karma, see here for a discussion.