Add `@sindresorhus/is` dependency

This will allow us to do runtime and, once we have TypeScript support,
compile-time type checks.

package.json

@@ -44,6 +44,7 @@
     "open-coverage": "open coverage/lcov-report/index.html"
   },
   "dependencies": {
+    "@sindresorhus/is": "^0.8.0",
     "archiver": "^2.1.1",
     "blob-util": "^1.3.0",
     "blueimp-canvas-to-blob": "^3.14.0",

yarn.lock

@@ -26,6 +26,10 @@
   version "0.7.0"
   resolved "https://registry.yarnpkg.com/@sindresorhus/is/-/is-0.7.0.tgz#9a06f4f137ee84d7df0460c1fdb1135ffa6c50fd"
 
+"@sindresorhus/is@^0.8.0":
+  version "0.8.0"
+  resolved "https://registry.yarnpkg.com/@sindresorhus/is/-/is-0.8.0.tgz#073aee40b0aab2d4ace33c0a2a2672a37da6fa12"
+
 "@sinonjs/formatio@^2.0.0":
   version "2.0.0"
   resolved "https://registry.yarnpkg.com/@sinonjs/formatio/-/formatio-2.0.0.tgz#84db7e9eb5531df18a8c5e0bfb6e449e55e654b2"