Break Middleman::Extension into its own file, mirroring the change on master
Be quiet about missing extras when middleman-core is used alone
Merge pull request #1190 from bhollis/middleman-core

Be quiet about missing extras when middleman-core is used alone
Merge remote-tracking branch 'origin/v3-stable'

Conflicts:
	middleman-core/lib/middleman-core/extension.rb