Platform specific filters for content and attributes

Add a way to add platform specific attributes and content.

**Attributes** example:

```
<Label ios:text="This is iOS specific" android:text="This is Android specific"></Label>
```

**Content** example:

```
<StackLayout>
    <ios><Label text="This is iOSspecific content"></Label></ios>
    <android><Label text="This is Android specific content"></Label></android>
</StackLayout>
```
