Merge codemirror config with user config.

Simplify customisation of codemirror without duplicating the all
codecell configuration.

Closes  #8038

IPython/html/static/notebook/js/cell.js

@@ -56,7 +56,6 b' define(['
         // superclass default overwrite our default
         
         this.placeholder = config.placeholder || '';
-        this.read_only = config.cm_config.readOnly;
         this.selected = false;
         this.rendered = false;
         this.mode = 'command';
@@ -76,7 +75,12 b' define(['
 
         // load this from metadata later ?
         this.user_highlight = 'auto';
-        this.cm_config = config.cm_config;
+
+        var class_conf_value = {};
+        if(this.class_config){
+            class_conf_value = this.class_config.get_sync('cm_config');
+        }
+        this.cm_config = utils.mergeopt({}, config.cm_config, class_conf_value);
         this.cell_id = utils.uuid();
         this._options = config;
 

IPython/html/static/notebook/js/codecell.js

@@ -163,7 +163,7 b' define(['
             notebook: this.notebook});
         inner_cell.append(this.celltoolbar.element);
         var input_area = $('<div/>').addClass('input_area');
-        this.code_mirror = new CodeMirror(input_area.get(0), this.class_config.get_sync('cm_config'));
+        this.code_mirror = new CodeMirror(input_area.get(0), this.cm_config);
         // In case of bugs that put the keyboard manager into an inconsistent state,
         // ensure KM is enabled when CodeMirror is focused:
         this.code_mirror.on('focus', function () {

IPython/html/static/notebook/js/textcell.js

@@ -129,7 +129,6 b' define(['
     };
 
     TextCell.prototype.unrender = function () {
-        if (this.read_only) return;
         var cont = Cell.prototype.unrender.apply(this);
         if (cont) {
             var text_cell = this.element;
@@ -230,10 +229,10 b' define(['
          */
         options = options || {};
         var config = utils.mergeopt(MarkdownCell, {});
-        TextCell.apply(this, [$.extend({}, options, {config: config})]);
-
         this.class_config = new configmod.ConfigWithDefaults(options.config,
                                             {}, 'MarkdownCell');
+        TextCell.apply(this, [$.extend({}, options, {config: config})]);
+
         this.cell_type = 'markdown';
     };