Display help as default Grunt task

If the user calls the Grunt CLI with no task (or with 'help' as the
task), then we can call grunt.help.display() to display the help screen.
Descriptions for Grunt task lists

Adds explanations for the 'deploy' and 'test' Grunt tasks for display on
the help screen.
Apply htmlmin options globally within Grunt

These aren't likelly going to change from target to target, as more
targets are added in the future (except possibly overriding individual
options on a per-target basis, maybe).