Tweak Maniphest custom fields

Summary:
  - Fix a bug where 'caption' didn't do anything.
  - Provide an abstract base implementation for extensions.
  - Add some documentation.
  - Expose aux fields via conduit.

Test Plan: Added some fields like "Dinosaur", "Kilograms" and "derp" on my local
install. Read documentation.
Reviewed By: jungejason
Reviewers: hunterbridges, jungejason, tuomaspelkonen, aran
CC: aran, philc, jungejason
Differential Revision: 785

src/docs/userguide/maniphest_custom.diviner

@@ -0,0 +1,66 @@
+@title Maniphest User Guide: Adding Custom Fields
+@group userguide
+
+How to add custom fields to Maniphest.
+
+= Overview =
+
+Maniphest provides some support for adding new fields to tasks, like an
+"cost" field, a "milestone" field, etc.
+
+NOTE: Currently, these fields are somewhat limited. They primarily give you a
+structured way to record data on tasks, but there isn't much support for
+bringing them into other interfaces (e.g., querying by them, aggregating them,
+drawing graphs, etc.). If you have a use case, let us know what you want to do
+and maybe we can figure something out. This data is also exposed via the Conduit
+API, so you might be able to write your own interface if you want to do
+something very custom.
+
+= Simple Field Customization =
+
+If you don't need complicated display controls or sophisticated validation, you
+can add simple fields. These allow you to attach things like strings, numbers,
+and dropdown menus to the task template.
+
+Customize Maniphest fields by setting ##maniphest.custom-fields## in your
+configuration. For example, suppose you want to add "Estimated Hours" and
+"Actual Hours" fields. To do this, set your configuration like this:
+
+  'maniphest.custom-fields' => array(
+    'mycompany:estimated-hours' => array(
+      'label'     => 'Estimated Hours',
+      'type'      => 'int',
+      'caption'   => 'Estimated number of hours this will take.',
+      'required'  => false,
+    ),
+    'mycompany:actual-hours' => array(
+      'label'     => 'Actual Hours',
+      'type'      => 'int',
+      'required'  => false,
+    ),
+  )
+
+Each array key must be unique, and is used to organize the internal storage of
+the field. These options are available:
+
+  - **label**: Display label for the field on the edit and detail interfaces.
+  - **type**: Field type, one of **int**, **string** or **select**.
+  - **caption**: A caption to display underneath the field (optional).
+  - **required**: True if the user should be required to provide a value.
+  - **options**: If type is set to **select**, provide options for the dropdown
+    as a dictionary.
+
+= Advanced Field Customization =
+
+If you want to add fields with more specialized validation, storage, or
+rendering logic, you can do so with a little work:
+
+  - Extend @{class:ManiphestAuxiliaryFieldSpecification} and implement
+    your specialized rendering, validation, storage, etc., logic.
+  - Extend @{class:ManiphestTaskExtensions} and return a list of fields which
+    includes your custom field objects.
+  - Set 'maniphest.custom-extensions' to the name of your new extensions
+    class.
+
+This is relatively advanced but should give you significant flexibility in
+defining custom fields.