Merge ICharAtlasRequest with ICharAtlasConfig

ICharAtlasRequest and ICharAtlasConfig need almost exactly the same set
of information, so it's simpler if we just merge the two types.

As an added bonus, this also adds devicePixelRatio to the config, which
helps guarantee that we won't ever accidentally end up with an atlas
using a different pixel ratio than we need.

src/shared/Types.ts

@@ -0,0 +1,13 @@
+/**
+ * Copyright (c) 2017 The xterm.js authors. All rights reserved.
+ * @license MIT
+ */
+
+export interface IColorSet {
+  foreground: string;
+  background: string;
+  cursor: string;
+  cursorAccent: string;
+  selection: string;
+  ansi: string[];
+}