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

tests/variable_declarator/string.js

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