Enhance `about_resource` to reference multiple files.

[chinyeungli]

Currently, the about_resource is only used for one reference. It should be able to have multiple references.

[chinyeungli]

This will also need to support multiple resource for ignored_resources

[chinyeungli]

By default, the ABOUT file will be generated side-by-side with the reference file; therefore, with multiple about_resource instances, such as the following:

about_resource	name
/about/ /about/test.csv /about/test.json	AboutCode

the tool will generate 3 ABOUT files. However, sometimes, this may not be the behavior that the user wants. As a result, we need to introduce an option to generate all ABOUT files in one place.