Support multiple invocations of install.

This patch allows `install` to be invoked several times for multiple extensions.

Example:

```javascript
var JSX = require('node-jsx')
JSX.install()
JSX.install({extension: '.coffee', additionalTransform: function ... )
```

If the multiple invocation style isn't to your liking I've also made a [multiple-extensions](https://github.com/sbrekken/node-jsx/tree/multiple-extensions) branch that expands options to achieve the same.