update to pandoc 1.18

Overview

Update the Pandoc interface to support several new configuration options, to support the use of the builder pattern, and to support returning the result of a Pandoc operation directly rather than always writing it to a file.

Details

Breaking changes

• update all builder-type methods on Pandoc to work as
• support returning a string directly from Pandoc's stdout. Previously, all output was written to file. The output location must be set using the OutputKind type passed as an argument to Pandoc::set_output. All configurations now return the new PandocOutput type.

Other changes

• add a Pandoc::show_cmd_line() method, which prints the generated command line call to stdout when it is run
• add a Pandoc::add_options() method, which works just like the existing Pandoc::add_option() method, but accepting a slice of PandocOption items rather than just one PandocOption

Under the hood

• update to use the JSON exchange format introduced in Pandoc 1.18 and added support for a smattering of other features from 1.18 and 1.17

Thanks to @chriskrycho for improving the api and writing up this changelog