Add object counter to seed fixtures

Use the key `:count` to define number of objects to create:

```yml
---
- type: user
  count: 3
  traits:
    - customer
```

Notice, that the counter can be set to 0 (or negative which is
the same as zero). This is useful to parameterize the seed:

```yml
---
- type: user
  count: <%= customers %>
  traits:
    - customer
```

lib/fixturama/seed.rb

@@ -7,8 +7,9 @@ def call(opts)
       type   = opts[:type].to_sym
       traits = Utils.symbolize_array opts[:traits]
       params = Utils.symbolize_hash  opts[:params]
+      count  = opts.fetch(:count, 1).to_i
 
-      FactoryBot.create(type, *traits, **params)
+      FactoryBot.create_list(type, count, *traits, **params) if count.positive?
     end
   end
 end

spec/fixturama/seed_fixture/_spec.rb

@@ -25,7 +25,8 @@
   end
 
   it "runs the factory", aggregate_failures: true do
-    expect(FactoryBot).to receive(:create).and_return(bar: 99, baz: 77, qux: 42)
+    expect(FactoryBot).to receive(:create_list).with(:foo, 1, :baz, qux: 42)
+    expect(FactoryBot).to receive(:create_list).with(:foo, 3, :bar, {})
 
     subject
   end

spec/fixturama/seed_fixture/seed.yml

@@ -1,7 +1,16 @@
 ---
 - type: foo
   traits:
-    - bar
     - baz
   params:
     qux: 42
+
+- type: foo
+  count: 3
+  traits:
+    - bar
+
+- type: foo
+  count: 0
+  traits:
+    - baz