[V3 Downloader] Allow use of prefix in install message (#1869)

* [V3 Downloader] Allow to use the prefix in install msg

* Update docs

* Let's do the same for repo addition

* Fix indent

* Use replace instead of format

* Update docs

docs/framework_downloader.rst

@@ -21,6 +21,9 @@ Keys common to both repo and cog info.json (case sensitive)
 - ``install_msg`` (string) - The message that gets displayed when a cog 
   is installed or a repo is added
   
+  .. tip:: You can use the ``[p]`` key in your string to use the prefix
+      used for installing.
+
 - ``short`` (string) - A short description of the cog or repo. For cogs, this info 
   is displayed when a user executes ``!cog list``
 

redbot/cogs/downloader/downloader.py

@@ -234,7 +234,7 @@ class Downloader:
         else:
             await ctx.send(_("Repo `{}` successfully added.").format(name))
             if repo.install_msg is not None:
-                await ctx.send(repo.install_msg)
+                await ctx.send(repo.install_msg.replace("[p]", ctx.prefix))
 
     @repo.command(name="delete")
     async def _repo_del(self, ctx, repo_name: Repo):
@@ -319,7 +319,7 @@ class Downloader:
 
         await ctx.send(_("`{}` cog successfully installed.").format(cog_name))
         if cog.install_msg is not None:
-            await ctx.send(cog.install_msg)
+            await ctx.send(cog.install_msg.replace("[p]", ctx.prefix))
 
     @cog.command(name="uninstall")
     async def _cog_uninstall(self, ctx, cog_name: InstalledCog):