Respect `externalLinkTarget` for GitHub Corner

[gluons]

Bug Report

Link target of repo's GitHub Corner should respect externalLinkTarget config.

Steps to reproduce

Set repo config and click GitHub Corner link.

What is current behaviour

Open repo in current window/tab.

What is the expected behaviour

Open repo in new window/tab by default.
(Because default value of externalLinkTarget is _blank)

Other relevant information

• Bug does still occur when all/other plugins are disabled?

• Your OS: Windows 10

• Node.js version: v10.15.3

• npm/yarn version: npm v6.9.0, Yarn v1.15.2

• Browser version: Firefox (Dev Edition) 67.0b15

• Docsify version: 4.9.4

• Docsify plugins: -