libraries: add libvterm

<https://www.leonerd.org.uk/code/libvterm/>
<https://launchpad.net/libvterm>

.gitmodules

@@ -264,3 +264,6 @@
 [submodule "libraries/rapidjson"]
 	path = libraries/rapidjson
 	url = https://github.com/Tencent/rapidjson/
+[submodule "libraries/libvterm"]
+	path = libraries/libvterm
+	url = https://github.com/wfrisch/libvterm-mirror

config.py

@@ -754,6 +754,24 @@ def sparse_paths(self):
         'src/unix/linux.c',
         'src/unix/core.c'
         ]),
+    Library('libvterm', [
+        "src/state.c",
+        "include/vterm.h",
+        "t/harness.c",
+        "src/vterm_internal.h",
+        "src/screen.c",
+        "src/vterm.c",
+        "src/parser.c",
+        "src/pen.c",
+        "src/keyboard.c",
+        "bin/vterm-ctrl.c",
+        "bin/unterm.c",
+        "bin/vterm-dump.c",
+        "src/encoding.c",
+        "src/unicode.c",
+        "include/vterm_keycodes.h",
+        "src/mouse.c",
+        ]),
     Library('libwebp', [
         "src/enc/vp8l_enc.c",
         "examples/cwebp.c",