feat(gatsby-plugin-feed): add match plugin option

[laysent]

Description

This PR is to propose a way to only include RSS feed in part of website, not all pages of it. gatsby-plugin-feed allows to customize the generated feed to only include part of the contents. However, the generated feed will be included in all pages by default, which might not be necessary in some cases.

It will be useful when for example blog is only part of the website, and there are many other pages which are not related to blog posts; or when there are different RSS feeds in website and each belongs to different part.

Adding an extra field match in feed array provides the ability to control where feed should be inserted when generating SSR result.

Related Issues

No issue related

[laysent]

@DSchau Hi, I noticed that this PR is labeled as awaiting author response. May I know if there is anything I can do here?

[LekoArts]

I think Dustin accidentally chose the wrong label so I think there's nothing needed from you at the moment. The team is out of office right now, so please have remind us of this PR next week. Thanks!

[wardpeet]

@laysent is there any reason why you chose a string and not directly went for a regex as an option?

[laysent]

@wardpeet I tried RegExp before, however it seems the RegExp option configured in gatsby-config.js cannot be passed to gatsby-plugin-feed. Looking at api-runner-ssr.js file generated under .cache folder, I found that RegExp provided in option has been transfered to {} there. Thus I then chose to use string instead, as it won't be transformed.

[laysent]

@LekoArts Hi, just to remind for this PR