add preferredPort (default 13331) to core opts

add a preferredPort to listen for hyperswarm connections on. the
preferredPort is smart enough that if it is blocked, hyperswarm will try
preferredPort+1, preferredPort+2 before giving up. see the hyperswarm code:
https://github.com/hyperswarm/network/blob/ad41b0c2ac19dbf92865e481282ba6ac6e513d57/index.js#L150-L162

this allows people with highly configured firewalls to port forward a
single port for cabal, which they can themselves configure in their
clients.

fix #98

index.js

@@ -54,6 +54,7 @@ function Cabal (storage, key, opts) {
   this.maxFeeds = opts.maxFeeds
   this.modKeys = opts.modKeys || []
   this.adminKeys = opts.adminKeys || []
+  this.preferredPort = opts.preferredPort || 13331
 
   if (!key) this.key = generateKeyHex()
   else {

swarm.js

@@ -16,7 +16,8 @@ module.exports = function (cabal, opts, cb) {
 
     const discoveryKey = crypto.discoveryKey(Buffer.from(cabal.key, 'hex'))
 
-    const swarm = hyperswarm()
+    const { preferredPort } = cabal
+    const swarm = hyperswarm({ preferredPort })
     swarm.join(discoveryKey, {
       lookup: true,
       announce: true