Add ConfigPanel superclass that can bind fields to config values

When you give a config field a `name` attribute based on a config key
path, such as 'editor.fontSize', it is automatically kept in sync with
the config value. You can also specify a `type` attribute of 'int' or
'float' to automatically convert the field value to a numeric type.
Specifying a type of 'string' is optional to signal no conversion.

src/app/config-panel.coffee

@@ -0,0 +1,24 @@
+$ = require 'jquery'
+{View} = require 'space-pen'
+
+module.exports =
+class ConfigPanel extends View
+  initialize: ->
+    @bindFormFields()
+
+  bindFormFields: ->
+    for input in @find('input[name]').toArray()
+      do (input) =>
+        input = $(input)
+        name = input.attr('name')
+        type = input.attr('type')
+        @observeConfig name, (value) -> input.val(value) if value
+        input.on 'change', ->
+          value = input.val()
+          config.set name, switch type
+            when 'int'
+              parseInt(value)
+            when 'float'
+              parseFloat(value)
+            else
+              value

src/app/editor-config-panel.coffee

@@ -1,13 +1,13 @@
-{View} = require 'space-pen'
+ConfigPanel = require 'config-panel'
 
 module.exports =
-class EditorConfigPanel extends View
+class EditorConfigPanel extends ConfigPanel
   @content: ->
     @div class: 'config-panel', =>
       @div class: 'row', =>
         @label for: 'editor.fontSize', "Font Size:"
-        @input name: 'editor.fontSize', size: 2
+        @input name: 'editor.fontSize', type: 'integer', size: 2
 
       @div class: 'row', =>
         @label for: 'editor.fontFamily', "Font Family:"
-        @input name: 'editor.fontFamily'
+        @input name: 'editor.fontFamily', type: 'string'