feat: support extracting datetime from post filename (#32)

closes #31 

* feat: support extracting datetime from post filename

* parse fileName once

packages/saber/lib/Source.js

@@ -61,11 +61,17 @@ module.exports = class Source {
 
   getPage(file) {
     const { api } = this
-    const slug = file.relative
-      // Remove leading _posts/
-      .replace(/^_posts\//, '')
-      // Remove extension
-      .replace(/\.[a-z]+$/i, '')
+
+    // A regex parsing RFC3339 date followed by {_,-}, and ended by some characters
+    const fileNameRegex = /^(((\d{4})-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])(T([01]\d|2[0-3]):([0-5]\d):([0-5]\d|60)(\.\d+)?(Z|(\+|-)([01]\d|2[0-3]):([0-5]\d)))?)(_|-))?(.+$)/
+    const parsedFileName = fileNameRegex.exec(
+      file.relative
+        // Remove leading _posts/
+        .replace(/^_posts\//, '')
+        // Remove extension
+        .replace(/\.[a-z]+$/i, '')
+    )
+    const slug = parsedFileName[16]
 
     const page = {
       attributes: {
@@ -99,8 +105,14 @@ module.exports = class Source {
     // These attributes depend on other attributes
     // And transformers can update the attributes
     // So we set them after the transformers
+
+    // Read createdAt from page attribute
+    // Or fallback to the date in fileName or the birthtime of the file
     page.attributes.createdAt = new Date(
-      page.attributes.createdAt || page.attributes.date || file.birthtime
+      page.attributes.createdAt ||
+        page.attributes.date ||
+        parsedFileName[2] ||
+        file.birthtime
     )
 
     page.attributes.type =