[fix-bug] image with same URL should not add twice more reliable id image tag fix

[Monirzadeh]

EmbededImage add same URL how many times it exist in HTML file that make unnecessary big file and some other problem. this PR download and add image to epub just once and reference all of same url image to the same file inside epub.

1. not download image twice
2. better id for images in opf file Fix wrong id for images with special chatrachter in name #70
3. fix content of image tag inside HTML to show correctly in eBooks reader.

[coveralls]

coverage: 87.947% (-0.05%) from 88.0% when pulling d9a1fa4 on Monirzadeh:fix-duplicate-image into 45f45a4 on bmaupin:main.

[Monirzadeh]

@bmaupin @owulveryck i get randomly same error test CI / test-mac-windows (macos-latest) in my pull request that my change is not related to that (it happen for second time). do you have any idea why this happen? i think it is not related to my change.

[Monirzadeh]

@bmaupin i think generate a valid id is better to fix filename to use as id.
if it is not problem for you i will update unit test for this PR. this fix #70
it is simpler and more reliable.

[Monirzadeh]

@bmaupin i think it is ready and fix 3 bug

1. download image twice
2. better id for images in opf file
3. fix content of image tag inside HTML to show correctly in eBooks reader.

if it is good to you i start update unit test.
UPDATE: do we need test image id unit test with new solution? if yes how we should that?