allow the stub content metadata to set a role attribute

[peetucket]

fixes #81

• pick up the role="transcription" file level attribute set in the goobi export to be passed into the content metadata generation script
• some refactoring of remove repetition of the file attributes checking methods
• be more intelligent about how file attributes set in the stub content metadata generation are passed through...previously if you had set any of the attributes, that is all you would get, now you can set one of them to override the default but still keep the rest for the defaults, e.g. before if the stub file has set preserve="true" but left off shelve and publish, you wouldn't get those at all in the generated content metadata. Now you will get the defaults for those values that you didn't indicate, but any values you set explicitly will be preserved.

[coveralls]

Coverage increased (+0.01%) to 95.225% when pulling dd5ea96e97d0b3e98ee60ba51a989feb3e1d01e6 on 81-parse-stub-content-metadata-ocr into 00ef4353a18cd89d34bf552954380ca24b8e0b19 on master.