Call “npm install” automatically if package.json changed (#505)

This adds a stamp file which is used to detect whether the `package.json`
file got updated since the last `npm install`.  If so, `npm install` is run
again to update all modules to the version described in `package.json`.
This happens as a dependency of only those modules which actually need some
npm-installed module.

Setting the corresponding make variable to the empty string disables the
feature, which is used by the `make` invocation in the `prepublish` script
inside `package.json` to avoid infinite loops.  It can also be used by
developers working in an environment with reduced connectivity, as long as
they know what they are doing.

package.json

@@ -33,7 +33,7 @@
   "bin": "cli.js",
   "scripts": {
     "test": "make lint test",
-    "prepublish": "make dist"
+    "prepublish": "make NIS= dist"
   },
   "dependencies": {
     "match-at": "^0.1.0"