Enabling a "false is empty" style check.

This is based off of pull request #10 from @athieriot.
Thanks for the idea!  I've implemented it as a flag and an environment
variable that can be enabled.  I've also added another test to handle
the environment variable.

tests/false-is-empty-arg.expected

@@ -0,0 +1 @@
+The user j.doe exists.

tests/false-is-empty-arg.sh

@@ -0,0 +1,4 @@
+#!/usr/bin/env bash
+
+cd "${0%/*}"
+USER=j.doe ADMIN=false ../mo --false false-is-empty-arg.template

tests/false-is-empty-arg.template

@@ -0,0 +1,4 @@
+The user {{USER}} exists.
+{{#ADMIN}}
+WRONG - should not be an admin.
+{{/ADMIN}}

tests/false-is-empty-env.env

@@ -0,0 +1,2 @@
+MO_FALSE_IS_EMPTY=yeppers
+someFalseValue=false

tests/false-is-empty-env.expected

@@ -0,0 +1 @@
+Works

tests/false-is-empty-env.template

@@ -0,0 +1,4 @@
+Works
+{{#someFalseValue}}
+  Never shown!
+{{/someFalseValue}}

tests/help.expected

@@ -6,3 +6,11 @@ environment variable.  You can use {{#VARIABLE}}content{{/VARIABLE}} to
 conditionally display content or iterate over the values of an array.
 
 Learn more about mustache templates at https://mustache.github.io/
+
+Simple usage:
+
+   mo [--false] [--help] [--source=FILE] filenames...
+
+--false       - Treat the string "false" as empty for conditionals.
+--help        - This message.
+--source=FILE - Load FILE into the environment before processing templates.