feat(instance): traverse up directory tree to find instance

[vikaspotluri123]

refs #800

• Try to go up the directory tree to find a valid Ghost instance
• Can be disabled with global config ('traverse_dirs')
  • We need to determine if there's a better way of doing this, or how global config should work in general
  • I was thinking of adding a migration which prompts the user, but that feels overkill
• I want to get a code review before I write the tests 😄

[coveralls]

Coverage remained the same at 99.925% when pulling 2198d18 on vikaspotluri123:feat/find-instance into 4df32ed on TryGhost:master.

[acburdine]

couple of super minor comments, otherwise looks good