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.
function _vendorsList(vendors, ignore = false) {
  let vendorsList = ''
  let sign = (ignore) ? '-' : '+'

  for (let vendor of vendors) {
    vendorsList += ` ${sign} ${vendor}`
  }

  return vendorsList
}