Simple update syntax

The Micropub spec only allows updates via JSON POST requests, in order to specify whether the update is replacing, adding or removing values in the lists. However, this leaves room for a simpler update syntax for the simple case of replacing values.

For example, a POST request that contains action=update and a url indicates that it is an edit instead of a create request. The remaining POST body parameters will replace any existing values of those properties. Only the fields specified will be changed, no values will be removed.

```
POST /micropub

action=update
&url=http://example.com/post/1000
&name=Hello+Moon
```

Note that because there is no way to indicate whether a value should be replaced or added, this is not useful for requests to add tags or add syndication URLs.

Start by reviewing the Micropub specification and the proposed form-encoded POST example in this issue. Before implementation, establish where the extension is documented or tested and get agreement on the replacement semantics, including which fields are changed and how unsupported add operations are handled.

Written by the indexing model from the issue text.