Remove elements with `.instapaper_ignore` class

[teddybradford]

Some feeds make use of Instapaper's customized parsing features to hide content such as pullquotes from Instapaper feeds. It would be nice if NetNewsWire supported hiding these elements as well.

This means stripping out elements that have the class name .instapaper_ignore, which would improve the reading experience for feeds that make use of such class.

An example feed is https://reallifemag.com/feed/ where the pullquotes are regular <p> tags with the .instapaper_ignore class. In NetNewsWire these appear unformatted as if they were part of the regular flow of the article rather than as pullquotes; for example:

…euphemistically, a form of labor that could be exploited. Ideally, the prosumer wouldn’t understand their contributions as labor at all but experience it as affirming interactivity. With the new media technology, the labor of consumption was inseparable from the enjoyment one took in consuming: Consumer ideas and behavior could be “captured” as data as well as overtly elicited as “valued feedback” or as participation in enclosed fan communities.</p>

<div class="pull-quote left-pull instapaper_ignore">
  <p>A creator is someone whom platforms can put to use</p>
</div>

<p class="p1">Some marketers touted co-creation as liberatory, on the questionable basis that consumers were oppressed by top-down advertising and limited choices, and that the…