Support attaching URLs to link to other content

[davidjgoss]

Problem

In Cucumber, attachments work by providing the attachment content to Cucumber which then includes it in the messages.

But sometimes you might just want to link to content that exists elsewhere but is relevant to this test case/step. Maybe it's a dump of logs, or some screenshots, or something else - anything addressable by a URL. Currently there's no way to do this. The schema for attachments has a url field, but this is intended for externalising regular attachments as a post-processing step (as done in cucumber/html-formatter#281) rather than linking to already-external content.

Proposal

We can use the existing sort-of-standard MIME type text/uri-list for this. The idea is to:

• Provide a link function in Cucumber implementations that acts as a shorthand for attaching a URL with the above MIME type
• In formatters - particularly the HTML formatter - render those attachments as a simple link

This is a similar approach to what we've done before with log.