Allow delayed loading of three.js

[1v9]

See #932

[1v9]

In fact I have seen too much blogs based on Github Pages enable features like three.js and they all sucks because the network condition between Github CDN and China is a mess.

To avoid blocking render, I thought of three ways:

• use hexo filter to operate DOM and put <script> in the bottom.

• modify the position of <script> in our swig template.

• use JavaScript, like this PR.

Surely this PR is not good enough, for wider use maybe hexo filter is best.

[jiangtj]

How about defer and async in script?
http://www.w3school.com.cn/tags/att_script_defer.asp
http://www.w3school.com.cn/tags/att_script_async.asp

[1v9]

@jiangtj

Yeah that's another method (before Domcontentloaded) but async is wrong.

<script defer src="/path/to/three.min.js"></script>
<script defer src="/path/to/three-waves.min.js"></script>

[1v9]

{% include '_scripts/three.swig' %} or {% include '_third-party/three.swig' %} or other position?