Extensions no longer have to be express apps.

This is a breaking change.
Update docs to reflect changes
Change "main" to "extension"
Update docs
Update docs
Improve extension logging when mounting
Fix typo in comment
Update docs
Fix relative link in extensions.md
Fix relative link in nodecg.json.md
Wording change in nodecg.json.md
Merge pull request #44 from nodecg/better-extensions

Better extensions