option "copy-external-resources": default value and consistent name

* dtbook-to-zedai: changed name of option from 'copy-images' to 'copy-external-resources'
* dtbook-to-zedai.convert: set default value of option 'copy-external-resources' to 'true'
Merge pull request #18 from daisy-consortium/dtbook-to-zedai.copy-external-resources.fix

option "copy-external-resources": default value and consistent name