fix: only add space to text, not links

[curbengh]

This is alternative approach to #8, in an attempt to address #4, #5.

This approach assumes each sentence is unique and only modifies text in limited html tags, i.e. <p> and <h1-3>

How to test

package.json
- "hexo-filter-auto-spacing": "^0.2.1"
+ "hexo-filter-auto-spacing": "curbengh/hexo-filter-auto-spacing#cheerio"

Demo: https://github.com/curbengh/hexo-testing/blob/master/source/_posts/test-pangu.md https://curbengh.github.io/hexo-testing/2018/10/05/test-pangu/

cc: @NanerLee, @geekrainy, @LeoJhonSong