Support ignore flag

[DenisGorbachev]

This input:

//! ```ignore
//! // If the code block has "ignore" flag, it is not processed, and the lines starting with "#" are not hidden
//! # // You can see me
//! ```

should result in this output:

```rust
// If the code block has "ignore" flag, it is not processed, and the lines starting with "#" are not hidden
# // You can see me

[msrd0]

Thanks for your PR. Please remove all unrelated changes from this PR and only implement ignore flags and nothing else as part of this PR.

[DenisGorbachev]

Done, it's clean now

[msrd0]

Thanks