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.

test/errors-spec.js

@@ -286,7 +286,7 @@ describe("environments.js:", function() {
         });
         it("rejects incorrectly scoped \\end", function() {
             expect("{\\begin{matrix}1}\\end{matrix}").toFailWithParseError(
-                   "Expected & or \\\\\ or \\end at position 17:" +
+                   "Expected & or \\\\ or \\end at position 17:" +
                    " …\\begin{matrix}1}̲\\end{matrix}");
         });
     });

test/katex-spec.js

@@ -2384,8 +2384,8 @@ describe("A macro expander", function() {
 
     it("should expand the \\overset macro as expected", function() {
         expect("\\overset?=").toParseLike("\\mathop{=}\\limits^{?}");
-        expect("\\overset{x=y}{\sqrt{ab}}")
-            .toParseLike("\\mathop{\sqrt{ab}}\\limits^{x=y}");
+        expect("\\overset{x=y}{\\sqrt{ab}}")
+            .toParseLike("\\mathop{\\sqrt{ab}}\\limits^{x=y}");
         expect("\\overset {?} =").toParseLike("\\mathop{=}\\limits^{?}");
     });