Pr/filename fixes

[boriselec]

1. fix trimming error (when char takes more than 1 byte) OSError: [Errno 36] File name too long: 'output/A_comfortable_and_attractive_chair._Can_be_used_at_tableswork_stationsandelsewhere.在这件家具上雕刻的内容刻画的是MadelineLandry正在精确着陆的程序中以空投舱方式撞向地面她坚强地微笑着作品给人一种几何风气息并使用了动态布局这件steps_50scale_7.50seed_519762248n_1.png'

see: https://stackoverflow.com/a/24839091/2513279

2. fix file creation on illegal filename (like '/' or so)

see: https://stackoverflow.com/a/55101759/2513279

3: refactor to print generated files

[fboulnois]

Thank you for the bug report, I used a slightly different approach:

• I removed invalid characters using re.sub
• I did truncation after removing invalid characters since this makes the prefix smaller and may fit in 170 characters
• I didn't include the changes which print out filenames to minimize output to the console.

The fixes are in #79 .