[TIDOC-2011] Support add-on YAML files

Testing:
Create an add-on YAML file.  See example below
Build the docs with the add-on file by specifying the `-a` options and passing it the directory containing the add-on file.
`node docgen.js -a /PATH/TO/ADDON/FILE`
Docs should merge and incorporate the add-on file changes.
With the example file:

  * Titanium.UI.Window.hideNavBar method should show support for Windows Phone platform and add the sentence to the description.
  * For Button class, nothing should change
  * For Label class, Windows Phone platforms should appear as supported for the class.  Most APIs of the class will not show Windows Phone as supported since most APIs are inherited from another class and some are platform specific.
  * Titanium.UI.backgroundColor property should show support for Windows Phone platform, version should be 3.6.1, and note should be added to the description.
  * Titanium.WindowsPhone and Titanium.WindowsPhone.Foo class should appear in docs.

~~~
---
name: Titanium.UI.Window

methods:
  - name: hideNavBar
    description: |
        Windows Phone does not yet utilize this method.
    platforms: [windowsphone]

---
name: Titanium.UI.Button
exclude-platforms: [windowsphone]
---
name: Titanium.UI.Label
platforms: [windowsphone]
---
name: Titanium.UI
properties:
  - name: backgroundColor
    description: |

        **Windows Phone Platform Notes**

        On Windows Phone, setting this property makes every window the same color.
    platforms: [windowsphone]
    since: 3.6.1
---
name: Titanium.WindowsPhone
summary: Windows Phone is great!
platforms: [windowsphone]
---
name: Titanium.WindowsPhone.Foo
summary: Converts a bar to a baz.
platforms: [windowsphone]
since: '3.6.2'
properties:
  - name: fooBar
    summary: The bar to convert to a baz.
    type: String

methods:
  - name: convert
    summary: Converts a bar to a baz.
    parameters:
      - name: bar
        summary: FooBar object to convert
        type: FooBar
    returns:
        type: FooBaz
~~~