[Duplicate] External storage support

[Scapy47]

Please, follow all of these checks (Issues that do not will be canceled):

• [x] This feature is not implemented and does not have opened/closed Issues.
• [x] I propose ONLY one feature at a time. One feature - one Issue.
• [x] I use English. This repository's language is English.
• [x] I properly filled the template below(not necessarily all parts, but at least first two).
• [x] I did not use any placeholder such as [Feature] or [Bug] in the title of the Issue.

Why you want this feature to be implemented? Please describe.

I have a SD card and a pen-drive that is filed with books. And I want to use that with your app

Describe the feature you'd like to be implemented

There should be option in setting where user can select which directory app will use to source books from using Android file/folder selection screen (sorry if it's wrong terminology I am from JavaScript ecosystem)

Additional context

I was looking for E-PUB reader for long time I found your app today. I like your app but I store my books in separate storage device so I can't use it for now

[Acclorite]

Duplicate of #78. Please, check all marks.