Issues/89 help wanted priority regex

[manekenpix]

Closes #89

Added checking if an issue is labelled help wanted using regex:

helpWantedRegExp = /help( |-)?wanted/i;
hasHelpWanted = (label) => {
  return helpWantedRegExp.test(label.name);
}

Also added checking if PRs have a priority label and, if not, adding a default one priority: medium.