Provide a way to serialize the header prefixed with a comment character

#### What version of the `csv` crate are you using?

1.1.6

#### Briefly describe the question, bug or feature request.

Serialize the header prefixed with a comment character (eg. '#')

#### Include a complete program demonstrating a problem.

```rust
use std::fs::File;
use std::io::Write;

use serde::Serialize;

#[derive(Debug, Clone, Serialize)]
pub struct Partition {
name: String,
#[serde(rename="type")]
partition_type: u32,
subtype: u32,
offset: u32,
size: u32,
flags: u32,
}

fn main() {
let partitions = vec![
Partition {
name: "test".to_owned(),
partition_type: 1,
subtype: 1,
offset: 1,
size: 1,
flags: 1,
},
Partition {
name: "test2".to_owned(),
partition_type: 1,
subtype: 1,
offset: 1,
size: 1,
flags: 1,
},
];
let mut wrt = csv::Writer::from_writer(std::io::stdout());
partitions.iter().for_each(|partition| wrt.serialize(partition).unwrap());
wrt.flush().unwrap();
}
```
#### What is the observed behavior of the code above?

`name,type,subtype,offset,size,flags`

#### What is the expected or desired behavior of the code above?

`#name,type,subtype,offset,size,flags`