feature enhancement: add export-package to ask-init to download skill-packages from smapi

[CamdenFoucht]

Description of changes:

Adding a simple export-package option to the ask init command.

I have recently been working with a team creating widgets for alexa skills and they are heavy users of ask init. But after they use ask init and commit changes, they also update the skill via the developer console.

This causes an issue because those changes aren't stored in git. So I've added an export-package option that will update ask init to download files from smapi alongside cloning the git repo.

Before this option, the ask init command would only download skill-package assets from smapi if a skill-package folder didn't exist in git. So if a developer does commit the skill-package folder, it will no longer use smapi export-package. Now when ask init --hosted-skill-id <id> --export-package is used, it will always download the skill schema from smapi so developers can recommit these files if wanted.

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