code: function sign (privateKey, data) {
    const pair = ECPair.fromWIF(privateKey)
    return btcMessage.sign(data, pair.d.toBuffer(32), pair.compressed).toString('base64')
  }
docs: Signs data with a bitcoin signature
  @param {string} privateKey - Bitcoin private key in WIF formate
  @param {string} data - Data to sign
  @return {string} - Base64 encoded signature