[Feature request] Add "Minify" and "Clear" buttons

[FluorescentHallucinogen]

What is the actual behavior?

Currently, GraphQL Playground has only "Prettify" button:

What is the expected behavior?

GraphQL Playground should also has "Minify" and "Clear" buttons:

[martin-888]

Minify functionality could be done by graphql-prettier as it's proposed here #999 .