Prefetch support

Closes #3 (closed).

README.rst

@@ -70,3 +70,8 @@ Options:
   Path to the PEM-encoded public client certificate for https with
   certificate-based client authentication. If this is specified,
   ``--client-key`` must be specified as well.
+
+* --prefetch
+
+  Download all files from the remote CAS server on startup to reduce access
+  latency.