[IDEA] Add switch to ignore extension when checking for duplicates

[Kesmy]

This could be an alternative to #25 and #26. A parameter to ignore the extension when downloading would allow for downloading a bundle that contains

Book 1 [epub] [mobi] [pdf]
Book 2 [mobi] [pdf] [pdf_hd]
Book 3 [cbz] [epub] [pdf] [pdf_hd]

Current:
Download 1: -f cbz

• Book 3.cbz

Download 2: -f pdf_hd

• Book 2.pdf
• Book 3.pdf

Download 3: -f epub

• Book 1.epub
• Book 3.epub

Ignoring Extension:
Download 1: -f cbz

• Book 3.cbz

Download 2: -f pdf_hd --compare-no-ext

• Book 2.pdf

Download 3: -f epub --compare-no-ext

• Book 1.epub