✨ Allow customizing "note provided" label

It's under `labels` in the config. It is possible to opt out with
`null`:

  labels:
    fragment-provided: ~

Resolves #40

README.md

@@ -30,6 +30,7 @@ exclude:
   - pyup-bot
 
 labels:
+  fragment-provided: change note detected  # default: `bot:chronographer:provided`, disable with `~`
   skip-changelog: skip news  # default: `bot:chronographer:skip`
 
 paths:  # relative modified file paths that do or don't need changelog mention

chronographer/event_handlers.py

@@ -156,6 +156,10 @@ async def on_pr(event):
         ))
 
     labels_config = repo_config.get('labels', {})
+    fragment_provided_label = labels_config.get(
+        'fragment-provided',
+        LABEL_PROVIDED,
+    )
     repo_skip_label = labels_config.get('skip-changelog', LABEL_SKIP)
 
     logger.info(
@@ -310,14 +314,14 @@ async def on_pr(event):
         else 'absent',
     )
 
-    if news_fragments_added:
+    if news_fragments_added and fragment_provided_label is not None:
         labels_url = f'{pull_request["issue_url"]}/labels'
         await gh_api.post(
             labels_url,
             preview_api_version='symmetra',
             data={
                 'labels': [
-                    LABEL_PROVIDED,
+                    fragment_provided_label,
                 ],
             },
         )