Implement a feature flags system.

* Scenarios contain a string map of feature flags. The flag names are the keys, and flag versions are the values, so a typical value might be "fixed" for bug fixes or for evolving features, "V1", "V2", etc.
* The game has a map of flags to lists of supported versions. The game can therefore signal that it supports a legacy behavior for a given feature flag. The last version in the list is considered to be this build version's default behavior.
* When launching a scenario, we check to make sure the game supports the scenario's required versions of its feature flags.
* When launching a replay, we make sure the game supports the feature flags that the version of the game that made the recording did.

Fix #555
Close #591

src/scenario/scenario.cpp

@@ -590,4 +590,14 @@ std::vector<town_entrance_t> cScenario::find_town_entrances(int town_num) {
 		}
 	}
 	return matching_entrances;
+}
+
+bool cScenario::has_feature_flag(std::string flag) {
+	return this->feature_flags.find(flag) != this->feature_flags.end();
+}
+
+std::string cScenario::get_feature_flag(std::string flag) {
+	std::map<std::string, std::string>::const_iterator iter = this->feature_flags.find(flag);
+	if(iter == this->feature_flags.end()) return "";
+	return iter->second;
 }

src/scenario/scenario.hpp

@@ -65,6 +65,10 @@ public:
 	location where_start,out_sec_start,out_start;
 	size_t which_town_start;
 	spec_num_t init_spec;
+	std::map<std::string,std::string> feature_flags;
+	bool has_feature_flag(std::string flag);
+	std::string get_feature_flag(std::string flag);
+
 	std::array<spec_loc_t,10> town_mods;
 	std::array<rectangle,3> store_item_rects;
 	std::array<short,3> store_item_towns;