code: function _vendorsList(vendors, ignore = false) {
    let vendorsList = ''
    let sign = (ignore) ? '-' : '+'
  
    for (let vendor of vendors) {
      vendorsList += ` ${sign} ${vendor}`
    }
  
    return vendorsList
  }
docs: Make a string with jspm vendors.
  @param {Array} vendors - List of dependencies used in jspm config
  @param {boolean} ignore - Indicate what sign use in vendor list (- or +).
  @returns {String} - jspm dependencies with sign - or + to use in arithmetic
  param of jspm shell command.