Support GitHub Issue Links

[HaaLeo]

Description

When referencing an GitHub issue in a repo's README.md or CHANGELOG.md like #3 or so, when I hover over that reference in the GitHub Web UI, I get a nice preview of the issue and can easily navigate to that issue by clicking the link.
However when the same readme is published to the vscode marketplace there is no such link created.

Proposal

1. Check whether the extension's repository is a GitHub repository
2. Search for github links using a regexp something like

    /\b\w*\/?\w*?#\d+\b/

   to match #​3 as well as Microsoft/vscode#​3
3. Replace
   • #3 with [#3](https://github.com/RepoOwner/RepoName/issues/3)
   • Microsoft/vscode#3 with [Microsoft/vscode#3](https://github.com/Microsoft/vscode/issues/3)