Added "allowBlank" as an option.

This pull request adds `allowBlank` as a new option. This will allow the input field to be left blank. And it won't default to #000000 on initialization.
I left the default value for `allowBlank` to `false` to keep backward compatibility in place. So when people update to the newer version, they don't get any surprises.
This is in reference to #24, #25 and #26.