VMB v1.6.1: Copy source code without --source flag and new default mod preview image

This update adds the ability to copy source code of a mod without the --source flag by setting copy_source_code in .vmbrc to true. --copy-source-code is now an alternative to the --source flag for the sake of consistency.

Note that you can temporarily reset flags by setting them to 0 (--source=0 means source code won't be copied even if copy_source_code is set to true).

Also, this update replaces the old .jpg template item preview image with a new better .png one by sL1bu. You'll need to change the preview file extension in .vmbrc to .png by hand or by running the following command:
vmb config --template_preview_image="item_preview.png"