Feautre/flexible thumbnail formats

[bdteo]

This pull request implements and documents a feature that allows setting the output format of the crops to a desired one among jpeg, webp, png and gif.

You can also see this issue: https://github.com/BKWLD/croppa/issues/212

This pull request also adds some tests to verify the feature works. It also adds some functional tests that verify that Handler@handle works.

Some comment typos are also fixed.

[sdebacker]

Hello, thank you for your work on this great pull-request. I began to test it and I see something to be fixed : the command php artisan croppa:purge doesn't remove the files generated with the format option set. Also, is it required to have "-format(jpg)" in the file name ? The extension is not sufficient ?