code: function encode (bits) {
    if (!bitString.isValid(bits)) throw new Error('baseURL#encode: bits not valid bitstring');
    if (bits.length % 6 !== 0) throw new Error('baseURL#encode: bits must be a multiple of 6');
  
    var result = '';
    for (var i = 0; i < bits.length; i = i + 6) {
      result += getChar(bits.slice(i, i + 6 ));
    }
    return result;
  }
docs: Encode a mod 6 bitstring into a baseURL string
  @param  {string} bits A mod 6 bitstring
  @return {string}      baseURL encoded string