fix: fix livereload deprecation warning

[jingsam]

Since livereload v0.6.3, we get the warning:

$ docsify serve ./docs --open
*** DEPRECATION WARNING *** The exts option will REPLACE extensions in 0.6.4. ***

As the author of livereload commented: in the next version, the exts option will REPLACE all the default extensions with only what you specify. That means exts: ['md'] will only watch markdown files. If we want to also watch default extensions, we should use extraExts: ['md'].

This PR upgrade livereload and use extraExts option to fix the warning.