Fix macOS Platform, Custom URL Scheme Handler, Polish Demos.

[chriszielinski]

• DownView did not work on macOS 10.14. It appears loadHTMLString(_:baseURL:) does not allow local file access, unlike on the iOS platform. On macOS, it now copies the necessary files to a temporary directory and uses loadFileURL(_:allowingReadAccessTo:).
• Added support for custom URL scheme handlers for macOS 10.13+ and iOS 11.0+. As well as a test case for it.
• Added a macOS demo application and polished the iOS demo.

I'm unsure of the changelog formatting, so have not updated that.

[chriszielinski]

Travis is attempting to test testCustomURLSchemeHandler() on macOS 10.12.* even though it's denoted @available(iOS 11.0, macOS 10.13, *). Not sure why... so I can either add an internal compiler directive to pass that case trivially, or update the osx_image Travis uses to a minimum of xcode9.3.

[iwasrobbed]

Hi @chriszielinski 👋

Thanks for all of these changes. Feel free to bump Travis's Xcode version to 9.4.1 and no worries about changelogs as that gets auto-generated.

I'll review these soon

[iwasrobbed]

This looks great overall, just a few changes to ensure things are speedy 👍

Thanks again!

[coveralls]

Coverage decreased (-2.1%) to 90.625% when pulling 35f0277 on chriszielinski:master into d753044 on iwasrobbed:master.

[iwasrobbed]

Looks great, thanks again @chriszielinski !!

[iwasrobbed]

Released as v0.6.1 👍 🚀