Make PluginInterface::GetBashTags() return a std::vector

Instead of a std::set. The tags are only iterated over, and uniqueness
and ordering are more of a concern for the UI.

Part of #68.

include/loot/plugin_interface.h

@@ -75,7 +75,7 @@ public:
    * @return A set of Bash Tags. The order of elements in the set holds no
    *         semantics.
    */
-  virtual std::set<Tag> GetBashTags() const = 0;
+  virtual std::vector<Tag> GetBashTags() const = 0;
 
   /**
    * Get the plugin's CRC-32 checksum.