config_parser: Add function for writing files

This will be used by the configuration writers shortly.

Change-Id: I6e72b181e365e34808c8e5e4790c446124dc13a3
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/src/config_parser.hpp b/src/config_parser.hpp
index 8ec693a..12ac417 100644
--- a/src/config_parser.hpp
+++ b/src/config_parser.hpp
@@ -190,6 +190,10 @@
      */
     void setFile(const fs::path& filename);
 
+    /** @brief Write the current config to a file */
+    void writeFile() const;
+    void writeFile(const fs::path& filename);
+
   private:
     fs::path filename;
     std::vector<std::string> warnings;