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.

src/Parser.js

@@ -847,7 +847,7 @@ class Parser {
             }
             return new ParseFuncOrArgument(
                 new ParseNode("ordgroup", expression, this.mode,
-                            firstToken, lastToken),
+                    firstToken, lastToken),
                 false);
         } else {
             // Otherwise, just return a nucleus, or nothing for an optional group