Support links to be opened in another tab

[liulex]

Sometimes we need some of the links to be opened in another browser tab, i.e. with target="_blank".

[QingWei-Li]

Which links?

[liulex]

For example, it's better to open links like [Issues](https://github.com/QingWei-Li/docsify/issues) in a new tab, so that the documention site is not replaced.

In the content, we are able to do this by using html:

<a href="https://github.com/QingWei-Li/docsify/issues" target="_blank">Issues</a>

But in the _sidebar.md, we cannot do this.

Oops, I just find out that in _sidebar.md, such code is also supported: 👍

  - <a href="https://github.com/QingWei-Li/docsify/issues" target="_blank">Issues</a>

I guess I have no problem then. 😃