Add new -i options, could prompt user before run migartions.

[xiaogaozi]

Sometimes we don't want to run migrations immediately. It's meaningful check migrations before actually run them. So I add the new -i option, for example:

$ node ./node_modules/mongodb-migrate -runmm -i down
  down : migrations/0010-add-index.js
  down : migrations/0005-add-some-collection.js
These migrations will be run, continue? [y/N]

[raiyankamal]

+1

[xiaogaozi]

@afloyd Can you see the PR sometime?