Add a public github repository browser feature

[MrHedmad]

One often needs to retrieve or inspect a remote repository file. We should add a cog that, for public repositories (private repos need token access, but we cannot store it not in plain text for now, so let's not do that), fuzzy-finds the files and:

• If no files match, say so and provide a link to the repo;
• If one file matches, provide a link and the content (cropped if need be) of the file;
• If multiple file match, provide a list of them with relevant links;

Additionally, it would be nice to implement:

• An optional grep pattern to match the file with;

Tracking

• [ ] Core implementation
• [ ] Grep support

[MrHedmad]

This feature would be really cool, but we need to access GitHub's API to do it, and it's not so easy to do. I therefore would implement it after 2.0.