- introduce additional placeholders supported by the template

- introduce placeholder support into `empty_template`
- introduce new placeholders for templates, update README with new placeholders

src/transform.ts

@@ -1,5 +1,6 @@
 import {PullRequestInfo, sortPullRequests} from './pullRequests'
 import * as core from '@actions/core'
+import {ReleaseNotesOptions} from './releaseNotes'
 import {
   Category,
   Configuration,
@@ -9,7 +10,8 @@ import {
 
 export function buildChangelog(
   prs: PullRequestInfo[],
-  config: Configuration
+  config: Configuration,
+  options: ReleaseNotesOptions
 ): string {
   // sort to target order
   const sort = config.sort || DefaultConfiguration.sort
@@ -96,10 +98,37 @@ export function buildChangelog(
     '${{UNCATEGORIZED}}',
     changelogUncategorized
   )
+
+  // fill other placeholders
+  transformedChangelog = transformedChangelog.replace(
+    '${{CATEGORIZED_COUNT}}',
+    categorized.size.toString()
+  )
+  transformedChangelog = transformedChangelog.replace(
+    '${{UNCATEGORIZED_COUNT}}',
+    uncategorized.length.toString()
+  )
+  transformedChangelog = fillAdditionalPlaceholders(
+    transformedChangelog,
+    options
+  )
+
   core.info(`ℹ️ Filled template`)
   return transformedChangelog
 }
 
+export function fillAdditionalPlaceholders(
+  text: string,
+  options: ReleaseNotesOptions
+): string {
+  let transformed = text
+  transformed = transformed.replace('${{OWNER}}', options.owner)
+  transformed = transformed.replace('${{REPO}}', options.repo)
+  transformed = transformed.replace('${{FROM_TAG}}', options.fromTag)
+  transformed = transformed.replace('${{TO_TAG}}', options.toTag)
+  return transformed
+}
+
 function haveCommonElements(arr1: string[], arr2: string[]): Boolean {
   return arr1.some(item => arr2.includes(item))
 }