Strip html from feed description/summary

[daiyi]

Using nunjucks striptags to remove formatting that was messing with rss/atom description/summary tag.

Before:

<h2 id="I-am-exactly-as-cool-as-I-was-before-I-started-writing-clojure"><a href="#I-am-exactly-as-cool-as-I-was-before-I-started-writing-clo

After:

I am exactly as cool as I was before I started writing clojure.I’ve had a short, short life as a programmer and I’ve spent a lot of it...

[coveralls]

Coverage remained the same at 100.0% when pulling dbc38e0 on daiyitastic:daiyi/update-rss2 into f1977f8 on hexojs:master.

[erikman]

I'm also interested in this, currently we can get a break in the middle of an html tag and this causes incorrect xml (e.g. the default hello world post from hexo):

<summary type="html">
<p>Welcome to <a href="https://hexo.io/" target="_blank" rel="noopener">Hexo</a>! This is your very first post. Check <a href="https://hexo
</summary>

[tomap]

Hi, this has been partially done in more recent version =>

{% if post.description %}
      {{ post.description }}
    {% elif post.intro %}
      {{ post.intro }}
    {% elif post.excerpt %}
      {{ post.excerpt | striptags }}

but if there are no excepts or intro or description, then the post content is used, but no striptags
I don't know how to modify the code to fix that

{% set short_content = striptags(post.content).substring(0, config.feed.content_limit) %} ?