Add a filter by account function to contributors

This allows to filter contributors by the account entries.
For example:

```js

Contributor.filterByAccount({site: 'github.com'}); // returns all
contributors with github account
Contributor.filterByAccount({site: 'github.com', username: 'bumi'});
// returns bumi

```

lib/contracts/contributor.js

@@ -34,6 +34,21 @@ class Contributor extends Base {
       });
   }
 
+  filterByAccount(search) {
+    return this.all().then((contributors) => {
+      const searchEntries = Object.entries(search);
+
+      return contributors.filter((contributor) => {
+        if (!contributor.accounts) { return false; }
+        return contributor.accounts.find((account) => {
+          return searchEntries.reduce((accumulator, searchValue) => {
+            return accumulator && account[searchValue[0]] === searchValue[1];
+          }, true);
+        });
+      });
+    });
+  }
+
   add(contributorAttr, callOptions = {}) {
     let json = ContributorSerializer.serialize(contributorAttr);
     // TODO: validate against schema