submodules: Add maddy markdown parser (header-only library)

Makefile

@@ -3,7 +3,7 @@
 CXXFLAGS=-std=c++17 -O0 -g -no-pie -pipe -MMD -Wall -Wextra
 RELEASE_CXXFLAGS=-std=c++17 -O3 -pipe -MMD -Wall -Wextra
 LDFLAGS=-lsqlite3 -lpthread -lcrypto -lstdc++fs -lseccomp
-INCLUDEFLAGS=-I submodules/sqlitemoderncpp/hdr -I submodules/cpp-httplib -I submodules/qssb.h
+INCLUDEFLAGS=-I submodules/sqlitemoderncpp/hdr -I submodules/cpp-httplib -I submodules/qssb.h -I submodules/maddy/include/
 
 CXX=g++