Lint expressions within ES6 Template Literals

Previously, expressions within ES6 template literals were not parsed, resulting in lint errors
caused by these expressions to be ignored. This CL corrects this by ensuring that template
expressions are parsed and checked.

Closes #1705
Merge branch 'issue-1705' of https://github.com/caitp/jshint into issue-1705

* 'issue-1705' of https://github.com/caitp/jshint:
  Lint expressions within ES6 Template Literals