Add the since_version badge to new features in the docs

[ianmcook]

In #263 @amoeba added a nice "since_version" badge.

In #297 I applied this to the --pre docs.

Let's go through the docs and add this to all the other docs sections describing features added since 0.1.0.

{{ since_version('v0.2.0') }}

• --json flag
• --quiet flag
• docs subcommand
• what else?