style mentioned issues (github eg)

[nylki]

As of now the .variable.issue.tag is using the blue of variable. maybe color the number blue as well.

for captured scopes see:

 {
    'match': '(?<=^|\\s|"|\'|\\(|\\[)(#)(\\d+)(?=[\\s"\'\\.,;\\)\\]])'
    'captures':
      '1':
        'name': 'variable.issue.tag.gfm'
      '2':
        'name': 'string.issue.number.gfm'
  }