Support for many image extentions and refactor results saving.

[leoromanovich]

Here I've added option to process folders with more, then one extention (like jpg, JPG, JPEG, png etc) in case insensitive way.

While refactoring I've tried to run image_demo.py and found, that it's failing, so, I've refactored that I little.

[leoromanovich]

@microsoft-github-policy-service agree

[zhmiao]

Thank you so much @leoromanovich for creating this PR and I am very sorry for not getting back to you earlier. This is a really good idea to and good addition to our code. Since we have make a couple changes since July, I will keep this pr open and see if I can add your changes to the current codebase if there are conflicts. Thanks again!