Fix escaping backslash in javascript lexer

[ashmaroli]

An attempt to resolve #828

Testing with the following code:

var foo = {
  a: "\test",
  b: "\nest",
  c: "\\test",
  d: "\\nest",
  e: "\\",
  f: "\\"
};

yields:

/cc @fireattack, @pyrmont, @dblessing

[pyrmont]

@ashmaroli Left a PR on your branch with some suggested fixes!