Added enable and disable breakpoint methods.

Per the enhancement request from https://github.com/linkedin/archetype/issues/99
considering there's so much reuse between `enabled_breakpoint` and `disable_breakpoint`, consider adding a private function that handles the bulk of this logic, something like...

```ruby
def enable_breakpoint(key)
  return enable_disable_breakpoint(key, 'enable')
end

def disable_breakpoint(key)
  return enable_disable_breakpoint(key, 'disable')
end

...

private
def enable_disable_breakpoint(key, toggle)
  if registered_breakpoints[key].nil? || helpers.is_null(registered_breakpoints[key])
    helpers.warn("[#{Archetype.name}:#{toggle}-breakpoint] a breakpoint for `#{key}` was not found.")
    return bool(false)
  end

  method = toggle == 'enable' ? 'push' : 'delete'
  breakpoints = Set.new(disabled_breakpoints) # this will ensure that there aren't any duplicate keys
  breakpoints.method(method).call(key)

  breakpoints = Sass::Script::Value::List.new(breakpoints, :space)
  environment.global_env.set_var('CONFIG_BREAKPOINTS_DISABLED', breakpoints)
end
```