Load custom Handlebar helpers located in "helpers" dir
added test for helper directory
added test for js extension on helper files
Load custom helper directory argument from command line
Make helper directory location default to sub-directory of template.
Fix variable name case.
Fix hard-coded helpers directory by replacing with the config variable.
Reserve -h for help flag instead of helpers directory.
Fix case of Sass in help text.
Merge branch 'handlebars-helpers' Fixes #117
Update cli options using new yargs syntax.
Update cli options in README.