mustache.js: use git-submodules instead of npm to include mustache.js

.gitmodules

+[submodule "src/3rdparty/mustache.js"]
+	path = src/3rdparty/mustache.js
+	url = https://github.com/janl/mustache.js/

package-lock.json

-{
-    "name": "xmpp-pane",
-    "version": "1.0.0",
-    "lockfileVersion": 1,
-    "requires": true,
-    "dependencies": {
-        "mustache": {
-            "version": "2.3.0",
-            "resolved": "https://registry.npmjs.org/mustache/-/mustache-2.3.0.tgz",
-            "integrity": "sha1-QCj3d4sXcIpImTCm5SrDvKDaQdA="
-        }
-    }
-}

package.json

-{
-    "name": "xmpp-pane",
-    "version": "1.0.0",
-    "description": "Tool to communicate with XMPP network services.",
-    "main": "index.js",
-    "dependencies": {
-        "mustache": "^2.3.0"
-    },
-    "devDependencies": {},
-    "scripts": {
-        "test": "echo \"Error: no test specified\" && exit 1"
-    },
-    "repository": {
-        "type": "git",
-        "url": "https://projects.adorsaz.ch/adrien/xmpp-pane"
-    },
-    "author": "Adrien Dorsaz <adrien@adorsaz.ch>",
-    "license": "GPL-3.0"
-}

mustache.js @ 3fddfb1c

+Subproject commit 3fddfb1cc91ef05bd8b76231ee84a89f8dd72da0