Add New document Feature

[yehia2amer]

The use-case for this feature is for example a user clicked on "New" and chose PDF document vocab1.pdf to be analyzed then he saved the session to .wordy file and after marking all the document in this file as known or unknown he want to add more words.
This feature can enable any user to add more documents to the .wordy file for example add vocab2.pdf unique words that don't already marked as known or unknown before.

As Discussed with @AdamCarroll in #41

[yehia2amer]

I know that we can ".wordy files as filter" but it's like dealing with a lot of files rather than only one. it would be better for me to have a way to merge them. or add incrementally. So that I can send only one file if i want to share with friend etc...