feat(plugins/code): add new code plugin (#526)

source/plugins/code/README.md

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+### ♐ Code snippet of the day
+
+> ⚠️ When improperly configured, this plugin could display private code. If you work with sensitive data or company code, it is advised to keep this plugin disabled. *Metrics* and its authors cannot be held responsible for any resulting code leaks, use at your own risk.
+
+Display a random code snippet from your recent activity history.
+
+<table>
+  <td align="center">
+    <img src="https://github.com/lowlighter/lowlighter/blob/master/metrics.plugin.code.svg">
+    <img width="900" height="1" alt="">
+  </td>
+</table>
+
+#### ℹ️ Examples workflows
+
+[➡️ Available options for this plugin](metadata.yml)
+
+```yaml
+- uses: lowlighter/metrics@latest
+  with:
+    # ... other options
+    plugin_code: yes
+    plugin_code_lines: 12               # Only display snippets with less than 12 lines
+    plugin_code_load: 100               # Fetch 100 events from activity
+    plugin_code_visibility: public      # Only display snippets from public activity
+    plugin_code_skipped: github/octocat # Skip github/octocat repository
+```