feat: import opml handler

[kimlimjustin]

Created OPML import handler, as discussed on #2870, by integrating https://github.com/danmactough/node-opmlparser solution into the packages/import-handler directory.

Yet, please note that due to insufficient up-to-date contributing docs and guidelines, this PR is made by literally reading the whole codebase manually, without actually running the application locally; this PR is only testified by running the mocha testing under the import-handler package.

That being said, please try this PR and please let me know if any errors occurred or if any changes should be made further to complete this pull request,

Thank you and hope that the OPML handler can be part of omnivore soon shortly!

[vercel]

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[kallekn]

This is a deal breaker. Omnivore looks nice, but I need to be able to import my feeds in order to use it.

[jacksonh]

Hey this is an interesting way of doing it. We actually do have an OPML API also but we haven't exposed it on the web, just because I don't think our handling of lots of RSS feeds is great yet.

[natarslan]

Following this! Would be great to be able to bulk import feeds (opml).