Add 'strict_variables' option to make Foil thrown exception when a required variable is missing

[gmazzap]

In #4 @skyosev proposed a 'strict_variables' option (similar to Twig one) that would be useful in development context.

My first thoughts is to add it and support 3 values:

• true -> thrown an Exception when a required var is not defined
• false -> resolve undefined vars as empty strings (current and default behavior)
• "notice" -> thrown a notice when a required var is not defined and resolve to empty string. This way it can be ignored using proper error_reporting setting.

[gmazzap]

Feature has been implemented in version 0.3.0