Add method to remove entries from a channel

Users may want to remove an entry from a channel for various reasons. Since [#4 tracking read state](https://github.com/indieweb/microsub/issues/4) means we now have referenceable entry IDs, we can use those to also remove entries from a channel.

Here is what I've implemented in Aperture:

```
POST /microsub

action=timeline
&method=remove
&entry=X
```

* `method=remove` vs `delete` because the server may not actually delete the entry, for example if the feed is added to another channel, or to allow undos
* `entry` or `entry[]` - may be a single value or multiple values

Start by reviewing issue #4 and the existing Microsub timeline method definitions. Define how POST /microsub with method=remove handles entry and entry[] values, including the non-destructive behavior described here. Done means the specification clearly documents the request parameters and expected removal semantics.

Written by the indexing model from the issue text.