Allow to break after = for strings (#1142)

This is not a general fix but it solves some issues that people are seeing where prettier goes > 80 columns.

Fixes #1141
Also fixes part of #1110

src/printer.js

@@ -3070,7 +3070,12 @@ function printAssignment(printedLeft, operator, rightNode, printedRight, options
     printed = indent(
       concat([hardline, printedRight])
     );
-  } else if (isBinaryish(rightNode) && !shouldInlineLogicalExpression(rightNode)) {
+  } else if (
+    (isBinaryish(rightNode) && !shouldInlineLogicalExpression(rightNode)) ||
+    rightNode.type === "StringLiteral" ||
+    (rightNode.type === "Literal" &&
+      typeof rightNode.value === "string")
+  ) {
     printed = indent(
       concat([line, printedRight])
     );

tests/flow/config_module_system_node_resolve_dirname/subdir/custom_resolve_dir/testproj2/__snapshots__/jsfmt.spec.js.snap

@@ -7,6 +7,7 @@ export var name: \\"subdir/custom_resolve_dir/testproj2\\" = \\"subdir/custom_re
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 // @flow
 
-export var name: \\"subdir/custom_resolve_dir/testproj2\\" = \\"subdir/custom_resolve_dir/testproj2\\";
+export var name: \\"subdir/custom_resolve_dir/testproj2\\" =
+  \\"subdir/custom_resolve_dir/testproj2\\";
 "
 `;

tests/variable_declarator/__snapshots__/jsfmt.spec.js.snap

@@ -0,0 +1,9 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`string.js 1`] = `
+"elements[0].innerHTML = '<div></div><div></div><div></div><div></div><div></div><div></div>';
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+elements[0].innerHTML =
+  \\"<div></div><div></div><div></div><div></div><div></div><div></div>\\";
+"
+`;

tests/variable_declarator/jsfmt.spec.js

@@ -0,0 +1 @@
+run_spec(__dirname, null, ["babylon", "typescript"]);

tests/variable_declarator/string.js

@@ -0,0 +1 @@
+elements[0].innerHTML = '<div></div><div></div><div></div><div></div><div></div><div></div>';