Plugin API Refactor: Filter and Theme Helpers

issue #769

- Refactor doFilter to allow returning a promise from a filter handler
  and to also return a promise itself
- Move the logic out of the registerThemeHelper calls and into their own methods so
  we could test them in isolation.
- Assign the server to the ghost instance so the initPlugins method can
  get access to it.
Read activePlugins from settings & improve error handling

issue #769

- activePlugins were being read from settings in two different ways, this has been simplified
- error handling has been improved so that plugins do not crash Ghost
- used full error messaging capabilities to make it easier to recover from errors
Merge pull request #931 from jgable/pluginApiChanges