Add support for setting NLSR prefix costs via configuration file and nlsrc

This change alters the nlsr.conf 'advertising' format, as the previous
setup used unnecessary keys which made dynamically adjusting the
stateful conf file difficult.

Incorporates code written by Yanbiao Li.

Change-Id: I3ed5d0a564099be8cc1389ee6acc6f2d04cef889
diff --git a/src/update/command-processor.hpp b/src/update/command-processor.hpp
index 1d25ec0..0973839 100644
--- a/src/update/command-processor.hpp
+++ b/src/update/command-processor.hpp
@@ -70,7 +70,7 @@
    *  \return tuple {bool indicating success/failure, message string}.
    */
   virtual std::tuple<bool, std::string>
-  afterAdvertise(const ndn::Name& prefix)
+  afterAdvertise(const ndn::Name& prefix, uint64_t cost)
   {
     return {true, "OK"};
   }