Start of the plugin system, based on pluggy (#210)

Uses https://pluggy.readthedocs.io/ originally created for the py.test project

We're starting with two plugin hooks:

prepare_connection(conn)

This is called when a new SQLite connection is created. It can be used to register custom SQL functions.

prepare_jinja2_environment(env)

This is called with the Jinja2 environment. It can be used to register custom template tags and filters.

An example plugin which uses these two hooks can be found at https://github.com/simonw/datasette-plugin-demos or installed using `pip install datasette-plugin-demos`

Refs #14

datasette/hookspecs.py

@@ -0,0 +1,15 @@
+from pluggy import HookimplMarker
+from pluggy import HookspecMarker
+
+hookspec = HookspecMarker('datasette')
+hookimpl = HookimplMarker('datasette')
+
+
+@hookspec
+def prepare_connection(conn):
+    "Modify SQLite connection in some way e.g. register custom SQL functions"
+
+
+@hookspec
+def prepare_jinja2_environment(env):
+    "Modify Jinja2 template environment e.g. register custom template tags"