what is prefer way to add relation and remove relation? I found example as below, but I did't find any document about the two methods.

 case 'ADD_AUTHOR_TO_BOOK':
        Book.withId(action.payload.bookId).authors.add(action.payload.author);  // add new entity ,  add relation?  this doesn't work.
        break;
    case 'REMOVE_AUTHOR_FROM_BOOK':
        Book.withId(action.payload.bo

Is your feature request related to a problem? Please describe.
Sometimes people need to run the same operation on multiple files.
For example: #23

Describe the solution you'd like
Allow -f, --file and -o, --out to be passed multiple times.

Dasel should then perform the operation (put or select) on all of the given files.

Describe alternatives you've considered