Update column pattern regex to incorporate special column comments

[drwl]

• Extracts AnnotationDiffGenerator out of FileAnnotator
• Updates the column pattern regex based on Annotation fails for columns with comments ctran/annotate_models#946
• Adds some tests for FileAnnotator

[drwl]

Note: working with column annotations are brittle due to the fact they're strings with varying white space. There's definitely work to be done to make this less brittle.