Add text block indicator that preserves newlines

**Using `"` at the beginning of a line works similarly to `|`, but newlines are preserved in the output (if `:pretty` is set to true).**

Reading Handlebars.js templates, for example, can be difficult when several consecutive lines of {{tags}} get consolidated into a single line. Since `:pretty` is intended to produce easier to read output, it’s useful to have the option to respect the newlines within a text block for this and other scenarios.

This is ignored (i.e. it behaves like `|`) if `:pretty` is set to false, as that seems like expected behavior.