Better snapshot formating

[Akiyamka]

Current snapshot md file contain json that look like plain text.
It can be much more readable with ```js around snap content.

Current	Can be

[novemberborn]

The output isn't strictly JavaScript though, for example this is how a buffer is snapshotted:

Buffer @Uint8Array [
  68656c6c 6f
]

[novemberborn]

Which happens to look OK enough in GitHub, but I'm not sure what other syntax bits the syntax highlighter may get weird about:

    Buffer @Uint8Array [
      68656c6c 6f
    ]