New plugin hook: register_output_renderer hook (#441)

Thanks @russss!

* Add register_output_renderer hook

This changeset refactors out the JSON renderer and then adds a hook and
dispatcher system to allow custom output renderers to be registered.

The CSV output renderer is untouched because supporting streaming
renderers through this system would be significantly more complex, and
probably not worthwhile.

We can't simply allow hooks to be called at request time because we need
a list of supported file extensions when the request is being routed in
order to resolve ambiguous database/table names. So, renderers need to
be registered at startup.

I've tried to make this API independent of Sanic's request/response
objects so that this can remain stable during the switch to ASGI. I'm
using dictionaries to keep it simple and to make adding additional
options in the future easy.

Fixes #440

docs/plugins.rst

@@ -551,3 +551,53 @@ The ``template``, ``database`` and ``table`` options can be used to return diffe
 The ``datasette`` instance is provided primarily so that you can consult any plugin configuration options that may have been set, using the ``datasette.plugin_config(plugin_name)`` method documented above.
 
 The string that you return from this function will be treated as "safe" for inclusion in a ``<script>`` block directly in the page, so it is up to you to apply any necessary escaping.
+
+.. _plugin_register_output_renderer:
+
+register_output_renderer(datasette)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+``datasette`` - Datasette instance
+    You can use this to access plugin configuration options via ``datasette.plugin_config(your_plugin_name)``
+
+Allows the plugin to register a new output renderer, to output data in a custom format. The hook function should return a dictionary, or a list of dictionaries, which contain the file extension you want to handle and a callback function:
+
+.. code-block:: python
+
+        @hookimpl
+        def register_output_renderer(datasette):
+            return {
+                'extension': 'test',
+                'callback': render_test
+            }
+
+This will register `render_test` to be called when paths with the extension `.test` (for example `/database.test`, `/database/table.test`, or `/database/table/row.test`) are requested. When a request is received, the callback function is called with three positional arguments:
+
+``args`` - dictionary
+    The GET parameters of the request
+
+``data`` - dictionary
+    The data to be rendered
+
+``view_name`` - string
+    The name of the view where the renderer is being called. (`database`, `table`, and `row` are the most important ones.)
+
+The callback function can return `None`, if it is unable to render the data, or a dictionary with the following keys:
+
+``body`` - string or bytes, optional
+    The response body, default empty
+
+``content_type`` - string, optional
+    The Content-Type header, default `text/plain`
+
+``status_code`` - integer, optional
+    The HTTP status code, default 200
+
+A simple example of an output renderer callback function:
+
+.. code-block:: python
+
+        def render_test(args, data, view_name):
+            return {
+                'body': 'Hello World'
+            }