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

datasette/utils.py

@@ -718,17 +718,16 @@ def get_plugins(pm):
     return plugins
 
 
-FORMATS = ('csv', 'json', 'jsono')
-
-
-async def resolve_table_and_format(table_and_format, table_exists):
+async def resolve_table_and_format(table_and_format, table_exists, allowed_formats=[]):
     if '.' in table_and_format:
         # Check if a table exists with this exact name
         it_exists = await table_exists(table_and_format)
         if it_exists:
             return table_and_format, None
+
     # Check if table ends with a known format
-    for _format in FORMATS:
+    formats = list(allowed_formats) + ['csv', 'jsono']
+    for _format in formats:
         if table_and_format.endswith(".{}".format(_format)):
             table = table_and_format[:-(len(_format) + 1)]
             return table, _format