Added options object and updated docs

Guide.md

@@ -27,6 +27,8 @@ User Accounts is a suite of packages for the [Meteor.js](https://www.meteor.com/
     * [Errors Text](#errors-text)
   * [Disabling Client-side Accounts Creation](#disabling-client-side-accounts-creation)
   * [Form Fields Configuration](#form-fields-configuration)
+  * [Extending Templates](#extending-templates)
+    * [Grouping Fields](#grouping-fields)
   * [CSS Rules](#css-rules)
 * [Wrapping Up for Famo.us](#wrapping-up-for-famo.us)
 * [Side Notes](#side-notes)
@@ -656,6 +658,7 @@ Each field object is represented by the following properties:
 | positiveValidation   | Boolean       |          | Specifies whether to display negative validation feed-back inside input elements. |
 | positiveFeedback     | Boolean       |          | Specifies whether to display positive validation feed-back inside input elements. |
 | showValidating       | Boolean       |          | Specifies whether to display a loding icon inside input elements while the validation process is in progress. |
+| options              | Object        |          | An object with custom properties that are made available inside the templates. This is used to customize the layout and style of forms through the use of [template-extension](https://github.com/aldeed/meteor-template-extension). See [Extending Templates](#extending-templates) |
 
 `displayName`, `placeholder`, and `errStr` can also be an [accounts-t9n](https://atmospherejs.com/softwarerero/accounts-t9n) registered key, in which case it will be translated based on the currently selected language.
 In case you'd like to specify a key which is not already provided by accounts-t9n you can always map your own keys. To learn how to register new labels, please refer to the official [documentation](https://github.com/softwarerero/meteor-accounts-t9n#define-translations).
@@ -956,6 +959,48 @@ AccountsTemplates.addFields([
 ```
 
 
+<a name="extending-templates"/>
+### Extending Templates
+
+With the [aldeed:template-extension](https://github.com/aldeed/meteor-template-extension) package, the built-in templates or sub-templates of any `user-accounts` UI package may be replaced by custom templates. The purpose is to create more sophisticated or specialized layouts or styling.
+
+Custom properties that hold information about the look of the form may be attached to the `options` object of a field. It may then be used to change the output while looping the fields. Adding a divider might look like this:
+
+```javascript
+AccountsTemplates.addField({
+  _id: "address",
+  type: "text",
+
+  // Options object with custom properties for my layout. At the moment, there are
+  // no special properties; it is up the developer to invent them
+  options: {
+    // Put a divider before this field
+    dividerBefore: true
+  }
+});
+```
+
+```html
+<template name="appAtInput">
+  {{#if options.dividerBefore}}<hr>{{/if}}
+
+  {{> Template.dynamic template=templateName}}
+</template>
+```
+
+```javascript
+Template.appAtInput.replaces("atInput");
+```
+
+
+<a name="grouping-fields"/>
+#### Grouping fields
+
+Grouping fields together is a special problem in regard to layout. The issue is creating some container markup *while* iterating over the fields (the templating engine of Meteor doesn't allow outputting an opening tag inside a loop without closing it in the same iteration).
+
+A solution to the problem is demonstrated in [this gist](https://gist.github.com/dalgard/a844f6569d8f471db9a7) (Semantic UI version).
+
+
 <a name="css-rules">
 ## CSS Rules
 

lib/core.js

@@ -154,6 +154,9 @@ FIELD_PAT = {
     trim: Match.Optional(Boolean),
     lowercase: Match.Optional(Boolean),
     uppercase: Match.Optional(Boolean),
+
+    // Custom options
+    options: Match.Optional(Object),
 };
 
 // Route configuration pattern to be checked with check