feat: add generate sitemap.txt

[uiolee]

• default generate sitemap.txt and sitemap.xml
• generator.js add judge path string or array
• template.js add extname(path) to chose template

sitemap:
    path:
    - sitemap.xml
    - sitemap.txt	#remove it to disable
    template_txt: sitemap.txt	#(optional)

[coveralls]

Coverage remained the same at 100.0% when pulling be922a3 on uiolee:master into 947e5c3 on hexojs:master.

[tomap]

Looks ok. In which case would you need a txt file?

[tomap]

Looks ok. I. Which case would you need a txt file?

[yoshinorin]

@uiolee
Thank you for your contribution :)

Would you please delete a comment out code?
And please update the options section in the README.

[yoshinorin]

@tomap

However, in addition to the XML protocol, we support RSS feeds and text files, which provide more limited information.
https://www.sitemaps.org/protocol.html#otherformats

According to the Sitemaps specifications, it can be provided as text file. IMHO we can accept this because this PR is based on the specifications.

[uiolee]

@tomap

Looks ok. I. Which case would you need a txt file?

txt file function as xml file. a few search engines support both xml and txt.

in my own case, i summit both txt and xml file to search engines.
on the other hand, i use txt file to include my site's urls, then i can summit they conveniently by API or manual.

[yoshinorin]

@uiolee

LGTM.
Thank you for your contribution :)

We are going to publish a new version.

[yoshinorin]

@uiolee

We published a new version.
Thank you :)

https://github.com/hexojs/hexo-generator-sitemap/releases/tag/3.0.0