Add ability limit write access to a subset of users

[ScreamingHawk]

This wiki has been perfect for my use case sans the ability to supply a list of moderators who are the only people able to edit the wiki pages. So I added it.

This pull requests adds another configuration parameter authorization.moderatorsFile which takes a path to a YAML file. The YAML file can contain a list of usernames and/or emails of moderators. When this configuration parameter is supplied, users can only edit the wiki if they are on this list.

Example moderators file:

usernames:
  - 'developer'
  - 'moderator'
emails:
  - 'jingouser'

[ScreamingHawk]

Related to #67. I agree with the "could blow out of scope" comment, so this implementation is an all or nothing solution when used.

[ScreamingHawk]

Closing in favour of new pull request #208 which uses a branch other than master