[CustomCom] Custom Command Parameters (#2051)

* [V3 CustomCom] Custom Command Parameters

Allows specifying more parameters for CC's via {0}, {1}, etc. that will be filled by the user invoking the CC. Python-style type hinting and attribute access is also allowed for Discord and builtin types.

> [p]cc add simple greet Hi, {0.mention:Member}!
> ...
> [p]greet zephyrkul
> Hi, @zephyrkul!

The bot will reply with the standard help messages if the cc is incorrectly executed.

> [p]greet me
> Member "me" not found

* black formatting

* check command failure

Only call the custom command if the faked command succeeded.

* misc fixes

1) don't str.strip all the time, it's not family-friendly and doesn't match transform_parameter
2) transform_arg now actually returns strings in every case
3) improve prepare_args parsing security
4) help parameters will show what type they expect
5) make linter less angery

* customcom documentation

I hate rst

* don't require repeated type hinting

If a parameter was type hinted previously, don't require it again.
Ex: `{0.display_name:Member}#{0.discriminator}` is now possible.

* add cog_customcom.rts to index

I despise rst

* don't enforce order

Allow type hinting and attribute access to be in either order.
Ex. `{0:Member.mention}` is now valid.

* clean up on_message

We're building context anyway, may as well use it.

* [doc] correct cog name

Cog class is named CustomCommands, not CustomCom

* minor on_message optimization

only build context if it's needed

* update cc_add docstring

Old one wasn't user-friendly. Replaced with a link to the new docs.
Link will not function until PR is merged and docs refreshed.

* [doc] change repeat to say

repeat is an audio command, use say in the example instead

* compare ctx.prefix to None

allows for null prefixes, which is a bad idea but who am I to judge

* address review

* raise error on conflicting colon notation

bugfix I was working on but failed to actually commit

docs/index.rst

@@ -20,6 +20,7 @@ Welcome to Red - Discord Bot's documentation!
     :maxdepth: 2
     :caption: Cog Reference:
 
+    cog_customcom
     cog_downloader
     cog_permissions