Currently ``` !commit abcd123 ``` generates a URL for that commit.

It would be good if say ``` !issue 1234 ``` would link to the issue/pr 1234.

Although pull requests show a URL like `geany/geany/pull/1234` it seems they will also be found with `geany/geany/issues/1234` the same as other issues, so the code just needs to be the same as `!commit` code but with URL base `https://github.com/geany/geany/issues%60 and limiting the argument to decimal numbers.