Added test for checkboxes, updated doc.

Checkboxes can have an unique name. However, in some circumstances two
or more checkboxes can share the same name attribute.

Author: rmariuzzo

README.md

@@ -8,7 +8,7 @@
 
 ## Features
 
-  - Package size is: 1.18KB (594B gzipped!).
+  - Package size is: 1.42KB (0.7KB gzipped!).
   - Supports all `<input />`s, checkboxes and `<select />`.
   - Supports `<select multiple />`.
   - Multiple bundles: [CJS, ESM and UMD](dist).