resolves #4 write execution result in cache directory

.gitignore

@@ -1 +1,2 @@
 node_modules/
+.cache/

spec/dynamic-notebook.spec.js

@@ -1,4 +1,6 @@
 /* global describe it */
+const fsp = require('node:fs/promises')
+const ospath = require('node:path')
 const chai = require('chai')
 const expect = chai.expect
 
@@ -88,6 +90,38 @@ NameError: name 'invalid_python' is not defined</pre>
 </div>
 </details>`)
     })
+    it('should write execution result when :dynamic-blocks-cache-result: is defined', async () => {
+      const input = `
+:dynamic-blocks:
+:dynamic-blocks-cache-result:
+
+[%dynamic,python]
+----
+print('hello')
+----
+`
+      const registry = asciidoctor.Extensions.create()
+      dynamicNotebookExt.register(registry)
+      const doc = asciidoctor.load(input, {extension_registry: registry})
+      const html = doc.convert()
+      expect(html).to.equal(`<div class="listingblock">
+<div class="content">
+<pre class="highlight"><code class="language-python" data-lang="python">print('hello')</code></pre>
+</div>
+</div>
+<details>
+<summary class="title">Results</summary>
+<div class="content">
+<div class="literalblock dynamic-py-result">
+<div class="content">
+<pre>hello</pre>
+</div>
+</div>
+</div>
+</details>`)
+      const executionResultData = await fsp.readFile(ospath.join(__dirname, '..', '.cache', 'e73b48e8e00d36304ea7204a0683c814-0.json'), 'utf8')
+      expect(executionResultData).to.equal(`{"success":true,"stderr":"","stdout":"hello\\n","id":"e73b48e8e00d36304ea7204a0683c814-0","code":"print('hello')"}`)
+    })
     it('should output more than one Plotly charts', () => {
       const input = `
 :dynamic-blocks:

src/dynamic-notebook-processor.js

@@ -1,5 +1,7 @@
 /* global Opal */
 const child_process = require('node:child_process')
+const fs = require('node:fs')
+const ospath = require('node:path')
 
 const conumRx = /\s*<i class="conum" data-value="[0-9]+"><\/i><b>[^>]+<\/b>/g
 const figShowRx = /fig.show\(\)/g
@@ -14,6 +16,7 @@ const ipythonTemplate = (pyCodes) => {
 from IPython.utils.capture import capture_output
 import json
 import sys
+import hashlib
 
 shell = InteractiveShell()
 results = []
@@ -24,10 +27,13 @@ ${pyCodes.map((pyCode, index) => {
     return `
 with capture_output() as io${index}:
     r${index} = shell.run_cell(${pyCode})
+    md5sum = hashlib.md5(${pyCode}.encode('utf8')).hexdigest()
     results.append({
         'success': r${index}.success,
         'stderr': io${index}.stderr,
-        'stdout': io${index}.stdout
+        'stdout': io${index}.stdout,
+        'id': f"{md5sum}-${index}",
+        'code': ${pyCode}
     })
 
 `
@@ -101,6 +107,16 @@ module.exports.register = function register(registry) {
               const exampleBlock = self.createExampleBlock(block, '', attrs, {'content_model': 'compound'})
               exampleBlock.setTitle('Results')
               const result = response[index]
+              let cacheResultDir = doc.getAttribute('dynamic-blocks-cache-result')
+              if (cacheResultDir !== undefined) {
+                if (cacheResultDir === '') {
+                  cacheResultDir = '.cache'
+                }
+                if (!fs.existsSync(cacheResultDir)) {
+                  fs.mkdirSync(cacheResultDir, { recursive: true })
+                }
+                fs.writeFileSync(ospath.join(cacheResultDir, `${result.id}.json`), JSON.stringify(result), 'utf8')
+              }
               let source = result.stdout.toString('utf8')
               if (result.success === false && block.hasAttribute("fail-on-error")) {
                 throw new ExecutionError(result.stderr.toString('utf8') + " " + result.stdout.toString('utf8'))