Link to non-md files under sub-directories

When link to files under sub-directories, you can use other types of link than md file link, and open it in a new window.

Unsupported cases

in file SUMMARY.md:

* [title A](path/to/file-a.md)
* [title B](path/to/file-a.html)
* [title C](path/to/file-a.pdf)

gitbook must parse the linked file in SUMMARY.md, so *.html, *.pdf files will fail the process.

And in file file-a.md:

* [title A](path/to/file-a.md)
* [title B](path/to/file-a.html)
* [title C](path/to/file-a.pdf)

the relative link will be opened in the same window. The file to be opened must get by ajax and then parsed in the browser in a specific way, so *.html and *.pdf link also fail to open.

Usage

in book.json:

{
    plugins: [ "otherlink" ]
}

in .bookignore:

*.html
*.pdf

to exclude the file types which you may use in SUMMARY.md.

and in your md files, write links of sub-directories' files in the below format:

* [title B](ref://path/to/file-a.html)
* [title C](ref://path/to/file-a.pdf)

just prefix the link ref://.