feat: enable the use of themes for simplified appearance customization

By using setting the `theme` prop to a packaged theme,
e.g., react-sortable-tree-theme-file-explorer,
a developer can achieve easy appearance customization without
having to resort to creating custom css/node renderer code themselves.
Themes can also be contributed by other members of the community.

Author: fritz-c

package-lock.json

@@ -57,7 +57,7 @@
     "react-dnd": "2.5.4",
     "react-dnd-html5-backend": "2.5.4",
     "react-dnd-scrollzone": "^4.0.0",
-    "react-virtualized": "^9.10.1"
+    "react-virtualized": "^9.11.1"
   },
   "peerDependencies": {
     "react": "^15.3.0 || ^16.0.0",
@@ -77,14 +77,14 @@
     "babel-loader": "^7.1.2",
     "babel-plugin-transform-object-rest-spread": "^6.26.0",
     "babel-polyfill": "^6.26.0",
-    "babel-preset-env": "^1.6.0",
+    "babel-preset-env": "^1.6.1",
     "babel-preset-react": "^6.11.1",
-    "cross-env": "^5.0.5",
+    "cross-env": "^5.1.0",
     "css-loader": "^0.28.7",
     "enzyme": "^3.1.0",
-    "enzyme-adapter-react-16": "^1.0.1",
-    "eslint": "^4.8.0",
-    "eslint-config-airbnb": "^16.0.0",
+    "enzyme-adapter-react-16": "^1.0.2",
+    "eslint": "^4.9.0",
+    "eslint-config-airbnb": "^16.1.0",
     "eslint-config-prettier": "^2.6.0",
     "eslint-loader": "^1.9.0",
     "eslint-plugin-import": "^2.7.0",
@@ -98,21 +98,21 @@
     "jest-enzyme": "^4.0.1",
     "json-loader": "^0.5.4",
     "node-sass": "^4.0.0",
-    "postcss-loader": "^2.0.7",
+    "postcss-loader": "^2.0.8",
     "prettier": "^1.7.4",
     "react": "^16.0.0",
     "react-addons-shallow-compare": "^15.6.2",
     "react-addons-test-utils": "^15.6.2",
     "react-dnd-test-backend": "^2.5.4",
     "react-dnd-touch-backend": "^0.3.15",
     "react-dom": "^16.0.0",
-    "react-hot-loader": "^3.0.0",
+    "react-hot-loader": "^3.1.1",
     "react-test-renderer": "^16.0.0",
     "rimraf": "^2.6.2",
     "sass-loader": "^6.0.6",
     "style-loader": "^0.19.0",
-    "webpack": "^3.7.1",
-    "webpack-dev-server": "^2.9.1",
+    "webpack": "^3.8.1",
+    "webpack-dev-server": "^2.9.2",
     "webpack-node-externals": "^1.6.0"
   },
   "keywords": [