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/core/Config.h

@@ -21,6 +21,7 @@
 #define RIPPLE_CORE_CONFIG_H_INCLUDED
 
 #include <ripple/basics/BasicConfig.h>
+#include <ripple/basics/base_uint.h>
 #include <ripple/protocol/SystemParameters.h>
 #include <ripple/protocol/RippleAddress.h>
 #include <ripple/json/json_value.h>
@@ -36,6 +37,7 @@
 #include <map>
 #include <string>
 #include <type_traits>
+#include <unordered_set>
 #include <vector>
 
 namespace ripple {
@@ -53,6 +55,8 @@ countSectionEntries (IniFileSections& secSource, std::string const& strSection);
 IniFileSections::mapped_type*
 getIniFileSection (IniFileSections& secSource, std::string const& strSection);
 
+class Rules;
+
 //------------------------------------------------------------------------------
 
 enum SizedItemName
@@ -257,6 +261,8 @@ public:
     boost::optional<boost::asio::ip::address_v4> rpc_ip;
     boost::optional<std::uint16_t> rpc_port;
 
+    std::unordered_set<uint256, beast::uhash<>> features;
+
 public:
     Config ();