eslint-plugin-flowtype & upgrade to eslint@4 (#849)

* Add eslint-plugin-flowtype. Fix #844

Using the recommended settings for plugin.
This involved adding some spaces to some existing union types.

* Upgrade to eslint@4, fix spotted bugs

Switched to indent-legacy to allow e.g. comments to have extra indents.

.eslintrc

@@ -1,5 +1,12 @@
 {
     "parser": "babel-eslint",
+    "plugins": [
+        "flowtype"
+    ],
+    "extends": [
+        "eslint:recommended",
+        "plugin:flowtype/recommended"
+    ],
     "rules": {
         "arrow-spacing": 2,
         "brace-style": [2, "1tbs", { "allowSingleLine": true }],
@@ -12,7 +19,8 @@
         "eol-last": 2,
         "eqeqeq": [2, "allow-null"],
         "guard-for-in": 2,
-        "indent": [2, 4, {"SwitchCase": 1}],
+        "indent": "off",
+        "indent-legacy": [2, 4, {"SwitchCase": 1}],
         "keyword-spacing": 2,
         "linebreak-style": [2, "unix"],
         "max-len": [2, 84, 4, { "ignoreUrls": true, "ignorePattern": "\\brequire\\([\"']|eslint-disable" }],
@@ -79,6 +87,5 @@
         "node": true,
         "browser": true
     },
-    "extends": "eslint:recommended",
     "root": true
 }