Add new --meta_format to `wp post meta` commands

Review the post meta list and post meta get command behavior and their documented --format options first. Resolve the proposed name for the key-value JSON format, then make both commands emit the format expected by the post create and post update meta_input examples and verify the documented command outputs.

According to the WP-CLI 1.5.0 Announcement:

The --meta_input option of the post create and post update commands now accepts JSON-formatted arrays, so you can add or update your post and its meta in one go:

$ wp post create --post_title='Title' --post_content='Content.' --meta_input='{"key1":"value1","key2":"value2"}
Success: Created post 123.

Unfortunately, there doesn't seem to be an equivalent export option for post meta list or post meta get.

wp post meta list 123 --fields=meta_key,meta_value --format=json will create [{"meta_key":"foo", "meta_value":"bar}] and not {"foo":"bar"} as shown in the examples for post update and post create.

post meta list

[--format=<format>]
Accepted values: table, csv, json, count. Default: table

post meta get

[--format=<format>]
Accepted values: table, json. Default: table

[--format=<format>]
Render output in a particular format.
---
default: table
options:
– table
– csv
– ids
– json
– count
– yaml

A suggestion has been made to add a new --format=meta_input or similar.

While the argument meta_input makes sense to solve this particular issue (based on import) it doesn't really describe the export type which is <key>:<value> JSON.

• --format=meta_input
• --format=json-key-value
• --format=json-key:value
• --format=jsonkv
• --format=json-kv