Make the choice of anchoring less magical

And hide the cache details from the default implementation that
doesn't use them.

h/static/scripts/annotator/anchoring/main.coffee

-if (window.PDFViewerApplication)
-  module.exports = require('./pdf')
-else
-  module.exports = require('./html')

h/static/scripts/annotator/guest.coffee

@@ -3,7 +3,6 @@ Promise = global.Promise ? require('es6-promise').Promise
 Annotator = require('annotator')
 $ = Annotator.$
 
-anchoring = require('./anchoring/main')
 highlighter = require('./highlighter')
 
 
@@ -25,7 +24,11 @@ module.exports = class Guest extends Annotator
     TextHighlights: {}
     TextSelection: {}
 
+  # Anchoring module
+  anchoring: require('./anchoring/html')
+
   # Internal state
+  anchors: null
   visibleHighlights: false
 
   html: jQuery.extend {}, Annotator::html,
@@ -36,11 +39,10 @@ module.exports = class Guest extends Annotator
       </div>
     '''
 
-  constructor: (element, options, config = {}) ->
+  constructor: (element, options) ->
     super
 
     this.anchors = []
-    this.anchoringCache = {}
 
     cfOptions =
       on: (event, handler) =>
@@ -164,7 +166,7 @@ module.exports = class Guest extends Annotator
         cache: self.anchoringCache
         ignoreSelector: '[class^="annotator-"]'
       }
-      return anchoring.anchor(target.selector, options)
+      return self.anchoring.anchor(target.selector, options)
       .then((range) -> {annotation, target, range})
       .catch(-> {annotation, target})
 
@@ -226,7 +228,7 @@ module.exports = class Guest extends Annotator
         cache: self.anchoringCache
         ignoreSelector: '[class^="annotator-"]'
       }
-      return anchoring.describe(range, options)
+      return self.anchoring.describe(range, options)
 
     setDocumentInfo = ({metadata, uri}) ->
       annotation.uri = uri

h/static/scripts/annotator/plugin/pdf.coffee

+extend = require('extend')
 Promise = global.Promise ? require('es6-promise').Promise
 Annotator = require('annotator')
 
@@ -8,8 +9,21 @@ class PDF extends Annotator.Plugin
   pdfViewer: null
 
   pluginInit: ->
-    @annotator.anchoringCache.pageText = {}
-    @annotator.anchoringCache.quotePosition = {}
+    cache = {
+      pageText: {}
+      quotePosition: {}
+    }
+
+    anchoring = require('../anchoring/pdf')
+
+    @annotator.anchoring = {
+      anchor: (selectors, options = {}) ->
+        options = extend({}, options, {cache})
+        return anchoring.anchor(selectors, options)
+      describe: (range, options = {}) ->
+        options = extend({}, options, {cache})
+        return anchoring.describe(range, options)
+    }
 
     @pdfViewer = PDFViewerApplication.pdfViewer
     @pdfViewer.viewer.classList.add('has-transparent-text-layer')