Add unofficial GitHub repos to the list of formulas that brew tracks,
    updates, and installs from.

def _tap(tap, runas=None):
    '''
    Add unofficial GitHub repos to the list of formulas that brew tracks,
    updates, and installs from.
    '''
    if tap in _list_taps():
        return True

    cmd = 'tap {0}'.format(tap)
    try:
        _call_brew(cmd)
    except CommandExecutionError:
        log.error('Failed to tap "%s"', tap)
        return False

    return True