Add npm support (#793)

Add package.json and npmignore to support installation of runtimes via
npm (address #711).

.npmignore

@@ -0,0 +1,19 @@
+CMakeLists.txt
+examples
+spine-as3
+spine-c
+spine-cocos2d-objc
+spine-cocos2dx
+spine-corona
+spine-csharp
+spine-js
+spine-libgdx
+spine-love
+spine-lua
+spine-monogame
+spine-sfml
+spine-starling
+spine-threejs
+spine-tk2d
+spine-unity
+spine-xna

package.json

@@ -0,0 +1,29 @@
+{
+  "name": "spine-runtimes",
+  "version": "3.5.35",
+  "description": "2D skeletal animation runtimes for Spine.",
+  "main": "spine-ts/build/spine-all.js",
+  "directories": {
+    "example": "examples"
+  },
+  "files": [
+    "spine-ts"
+  ],
+  "repository": {
+    "type": "git",
+    "url": "git@github.com:EsotericSoftware/spine-runtimes.git"
+  },
+  "keywords": [
+    "spine",
+    "runtimes",
+    "2d",
+    "skeletal",
+    "animation"
+  ],
+  "author": "Esoteric Software",
+  "license": "SEE LICENSE IN LICENSE",
+  "bugs": {
+    "url": "https://github.com/EsotericSoftware/spine-runtimes/issues"
+  },
+  "homepage": "https://github.com/EsotericSoftware/spine-runtimes#readme"
+}