Powershell: disable backticks in HereStrings and Single-Quoted strings

This disables backtick-escaping within all here-strings (single- and double-quoted) as well as in regular single-quoted strings as described by @ITFiend in his [comment](https://github.com/aramk/crayon-syntax-highlighter/issues/177#issuecomment-42261258) in #177.  Backtick-escaping will still work in double-quoted strings.