Add two optional options that are used to skip dialogs

[loban]

Users of the plugin (including me) may want to create a seamless experience for their users and not show any dialog prompts. This patch allows two additional options when calling the checkAppUpdate() function.

• skipPromptDialog (when true, then the plugin immediately and silently starts downloading an update when found)
• skipProgressDialog (when true, then the plugin does not show the progress bar dialog, and instead does it in the background)

This was inspired by pull request https://github.com/vaenow/cordova-plugin-app-update/pull/78, and incorporates the request to make it an optional feature.

[vaenow]

I have reviewed your code, it looks good. Thank you @loban for your a lot of contributions 🍻

[loban]

You're welcome! Glad to help. This plugin has been very useful in our current project.