Support Templating in Change Scripts

[theory]

Add support for running deploy/verify/revert scripts through Template::Tiny or Template Toolkit, reading variable settings from a YAML or JSON file. Read in the scripts, run them through the template engine, write out a temporary file, have the database client run the temp file, then delete it.

[theory]

If want to use command-line options, need to figure out how to support both this functionality and --set. Maybe require a file with variables set? What about support in the Sqitch configuration?