Allow for custom fallback tags.

This lets you pass an optional set of fallback tags into the react jsx transpiler. This may be useful

  -- transformer --

```javascript
  transform(... {
    fallbackTags: {
      MyCustomXML: {
        FooNode: true,
        BarNode: true
      }
    }
  });
```
  -- source --

```
  /** @jsx MyCustomXML */
  <FooNode><BarNode /></FooNode>
```
  -- compiled --

```
  /** @jsx MyCustomXML */
  MyCustomXML.FooNode(null, MyCustomXML.BarNode());
```
probably want to use `hasOwnProperty` instead of undefined property lookup to avoid gaming the system. We've had a few other fixes related to that.

eg `@jsx constructor`