Recognize escaped quote characters in strings

Recognize escaped single and double quote characters within their respective string types.

`'"'` was already recognized as a complete string, now so is `'\''`
`"'"` was already recognized as a complete string, now so is `"\""`

Unified `#string-double-quoted` and `#string-single-quoted` includes under a common `#string` clause.

Added examples to `template.handlebars`