Add blockHeader instructions and examples.

examples/html/bitcoin/blocks.html

@@ -30,6 +30,9 @@
         const blockRaw = await blocks.getBlockRaw({ hash });
         console.log(blockRaw);
 
+        const blockHeader = await blocks.getBlockHeader({ hash });
+        console.log(blockHeader);
+        
         const blockHeight = await blocks.getBlockHeight({ height: 0 });
         console.log(blockHeight);