Allow system cpptoml

meson.build:
When the 'wrap_mode' option is set to 'nodownload' use a system version
of cpptoml. This does not require using git and having a network
connection during build, which is important for Linux packaging
infrastructure.

subprojects/cpptoml.wrap:
Pin revision to last release tag (v0.1.1).

lib/wptoml/*.cpp:
Remove 'include/' prefix from all cpptoml related includes, at is not
required.

Closes #17

lib/wptoml/array.cpp

@@ -10,7 +10,7 @@
 #include <functional>
 
 /* CPPTOML */
-#include <include/cpptoml.h>
+#include <cpptoml.h>
 
 /* TOML */
 #include "private.h"