Implement support for inline JS code

[tadatuta]

Proposal:

b1
    `this.ctx.content`

to be converted to

{
    block: 'b1',
    content: this.ctx.content
}

But it's impossible to keep this.ctx.content as is inside JS object so it should be escaped somehow until it's stringified. Let's say it can be kept as

{
    block: 'b1',
    content: '%%%%%this.ctx.content%%%%%'
}

and then stringified as

`{
    block: 'b1',
    content: this.ctx.content
}`