add callback for embedding in other html game engine IDE, also fix webapp file saving and loading

[blurymind]

This adds a callback to allow yarn to be embedded in other web apps, such as gdevelop for example.

The callback sends the parent app yarn's document and data object - so as to allow for them to be controlled externally. That way the parent app can send data to yarn's data methods and open it upon yarn opening. It can also add a custom button that would save the data and close yarn.

[blurymind]

I also madeit possible to save and load files from the web app.

@charblar or @AlecHolowka will need to enable it from this git repo's settings in order to makeit available at: https://InfiniteAmmoInc.github.io/Yarn/app/

meanwhile, you can use it at my fork here https://blurymind.github.io/Yarn/app/

[blurymind]

to make it available, go to the settings tab https://github.com/InfiniteAmmoInc/Yarn/settings

and set source to master branch

I can now merge prs , but I can't change the repo's settings. I think only infiniteAmmo can :)

Then it will become fully live at https://InfiniteAmmoInc.github.io/Yarn/app/

we can then use it from a smarthphone even :p