Added customisable sharing via native share widget

Added customisable sharing via native share widget. This code uses the first two meta tags on each html page.

If you customise the javascript found in the getShareTitle and getShareURL methods in SharePage.m you can source the title and URL to share from anywhere in your html, limited only by the javascript you can execute.

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Based on the work from/fixes:
https://github.com/bakerframework/baker/issues/10