Add a way to clearly mark a variable as having no value

[brikis98]

There is some awkwardness in boilerplate, and Go in general, with how to identify when you want to set a variable to "no value". In other languages, this would be done using null or None, but in Go, while some types can be set to nil, most primitives can only be set to their "zero values" (e.g. "" for a string, 0 for an int, etc). This makes it hard to set "no value", both in the default parameter in a boilerplate.yml file and as a user when enter the value via interactive prompt or the --var and --var-file options.

Perhaps a simply, albeit not terribly elegant option is to simply define a placeholder that globally means "use the zero value for this type". A simple one would be something like __NO_VALUE__. This is unlikely to collide with any real value a user is likely to enter and pretty clearly communicates the intent. Example:

variables:
  name: Title
  default: __NO_VALUE__