feat: Add dynamic retrieval for client password

Adds option to specify a function for a client password. When the client
is connected, if the value of password is a function then it is invoked
to get the password to use for that connection.

The function must return either a string or a Promise that resolves to
a string. If the function throws or rejects with an error then it will
be bubbled up to the client.

lib/client.js

@@ -114,7 +114,26 @@ Client.prototype._connect = function (callback) {
 
   function checkPgPass (cb) {
     return function (msg) {
-      if (self.password !== null) {
+      if (typeof(self.password) === 'function') {
+        try {
+          self._Promise.resolve(self.password())
+            .then(pass => {
+              if (undefined !== pass) {
+                if (typeof(pass) !== 'string') {
+                  con.emit('error', new Error('Password must be a string'));
+                  return
+                }
+                self.connectionParameters.password = self.password = pass
+              }
+              cb(msg)
+            })
+            .catch(err => {
+              con.emit('error', err)
+            })
+        } catch (err) {
+          con.emit('error', err)
+        }
+      } else if (self.password !== null) {
         cb(msg)
       } else {
         pgPass(self.connectionParameters, function (pass) {