?_ttl= parameter and default_cache_ttl config

Refs #285, Closes #289

docs/config.rst

@@ -92,3 +92,12 @@ allow_sql
 Enable/disable the ability for users to run custom SQL directly against a database. To disable this feature, run::
 
     datasette mydatabase.db --config allow_sql:off
+
+default_cache_ttl
+-----------------
+
+Default HTTP caching max-age header in seconds, used for ``Cache-Control: max-age=X``. Can be over-ridden on a per-request basis using the ``?_ttl=`` querystring parameter. Set this to ``0`` to disable HTTP caching entirely. Defaults to 365 days (31536000 seconds).
+
+::
+
+    datasette mydatabase.db --config default_cache_ttl:10

docs/json_api.rst

@@ -164,6 +164,10 @@ The Datasette table view takes a number of special querystring arguments:
     long, for example if you want to implement autocomplete search but only if
     it can be executed in less than 10ms.
 
+``?_ttl=SECONDS``
+    For how many seconds should this response be cached by HTTP proxies? Use
+    ``?_ttl=0`` to disable HTTP caching entirely for this request.
+
 ``?_next=TOKEN``
     Pagination by continuation token - pass the token that was returned in the
     ``"next"`` property by the previous page.