Added the upload book from PC feature

[intity]

Please consider enabling the function to download a book file from a PC.

[maggie44]

A scan over the code, and it looks like this isn't 'uploading' as such, as it isn't creating a storage directory for the EPUB. Instead perhaps using 'Open' instead of 'Upload' would be more accurate terminology?

I think there should probably be a variable too that allows someone to enable or disable the feature, as it's a bit of a step away from the original viewer?