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/ParseError.js

@@ -11,11 +11,12 @@ import {Token} from "./Token";
  * about where in the source string the problem occurred.
  */
 class ParseError {
-    position: number|void; // Error position based on passed-in Token or ParseNode.
+    position: number | void;
+        // Error position based on passed-in Token or ParseNode.
 
     constructor(
-        message: string,         // The error message
-        token?: Token|ParseNode, // An object providing position information
+        message: string,           // The error message
+        token?: Token | ParseNode, // An object providing position information
     ) {
         let error = "KaTeX parse error: " + message;
         let start;

src/ParseNode.js

@@ -16,9 +16,9 @@ export default class ParseNode {
     // TODO: We should combine these to ({lexer, start, end}|void) as they
     // should all exist together or not exist at all. That way, only a single
     // void check needs to be done to see if we have metadata.
-    lexer: LexerInterface|void;
-    start: number|void;
-    end: number|void;
+    lexer: LexerInterface | void;
+    start: number | void;
+    end: number | void;
 
     constructor(
         type: string,       // type of node, like e.g. "ordgroup"

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

src/macros.js

@@ -216,7 +216,7 @@ defineMacro("\\dotsi", "\\!\\cdots");
 // amsmath doesn't actually define \dotsx, but \dots followed by a macro
 // starting with \DOTSX implies \dotso, and then \extra@ detects this case
 // and forces the added `\,`.
-defineMacro("\\dotsx", "\\ldots\,");
+defineMacro("\\dotsx", "\\ldots\\,");
 
 // \let\DOTSI\relax
 // \let\DOTSB\relax