get header-only library dependencies using git submodules

.gitmodules

@@ -0,0 +1,6 @@
+[submodule "submodules/sqlitemoderncpp"]
+	path = submodules/sqlitemoderncpp
+	url = https://github.com/SqliteModernCpp/sqlite_modern_cpp
+[submodule "submodules/cpp-httplib"]
+	path = submodules/cpp-httplib
+	url = https://github.com/yhirose/cpp-httplib