Add [features] section to .cfg:

This non-production config section allows features to be enabled
by listing their text descriptions, one line each, in the config
section titled "features".

NOTE: Feature names with leading or trailing whitespace, or
      containing an equals sign ('=') are not supported.

src/ripple/protocol/Feature.h

@@ -0,0 +1,39 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of rippled: https://github.com/ripple/rippled
+    Copyright (c) 2012, 2013 Ripple Labs Inc.
+
+    Permission to use, copy, modify, and/or distribute this software for any
+    purpose  with  or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL ,  DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#ifndef RIPPLE_PROTOCOL_FEATURE_H_INCLUDED
+#define RIPPLE_PROTOCOL_FEATURE_H_INCLUDED
+
+#include <ripple/basics/base_uint.h>
+#include <string>
+
+namespace ripple {
+
+/** Convert feature description to feature id. */
+/** @{ */
+uint256
+feature (std::string const& name);
+
+uint256
+feature (const char* name);
+/** @} */
+
+} // ripple
+
+#endif