Added Downloader cog (#786)

cogs/downloader/converters.py

@@ -0,0 +1,24 @@
+import discord
+from discord.ext import commands
+from .repo_manager import RepoManager
+from .installable import Installable
+
+
+class RepoName(commands.Converter):
+    async def convert(self, ctx: commands.Context, arg: str) -> str:
+        return RepoManager.validate_and_normalize_repo_name(arg)
+
+
+class InstalledCog(commands.Converter):
+    async def convert(self, ctx: commands.Context, arg: str) -> Installable:
+        downloader = ctx.bot.get_cog("Downloader")
+        if downloader is None:
+            raise commands.CommandError("Downloader not loaded.")
+
+        cog = discord.utils.get(downloader.installed_cogs, name=arg)
+        if cog is None:
+            raise commands.BadArgument(
+                "That cog is not installed"
+            )
+
+        return cog