Block tunes API

[johnpuddephatt]

It looks like the new Block Tunes API is unsupported by editorjs-php at present.

This adds 'tuning' data under the 'tunes' key so block data looks like the following:

"type": "paragraph",
"data": {
    "text": "Foo"
},
"tunes": {
    "textVariant": "small"
}