Add docsify version to $window.docsify object

[jhildenbiddle]

Fix #521

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[QingWei-Li]

I forgot to get the version before by Docsfiy.version

docsify/src/core/index.js

Line 41 in b3b6116

Docsify.version = '__VERSION__'

😂

[jhildenbiddle]

@QingWei-Li --

The altered implementation does not work. There is no version key available on the Docsify object. The goal is to have the version number accessible from the browser environment to allow plugins to behave differently based on the version.

An easy test is to try to access the version value from the console in a browser. With my implementation, the version was available via $docsify.version. With your altered implementation you've moved it to the Docsify object, so it should be availalbe at Docsify.version.