behead: add --flexible option

when reading CSV files to omit num col check for performance.

Also set writes to be always flexibile for even more performance.

src/cmd/behead.rs

@@ -7,6 +7,8 @@ Usage:
 
 Common options:
  -h, --help Display this message
+ -f, --flexible Do not validate if the CSV has different number of
+ fields per record, increasing performance.
  -o, --output <file> Write output to <file> instead of stdout.
 "#;
 
@@ -16,16 +18,18 @@ use crate::{config::Config, util, CliResult};
 
 #[derive(Deserialize)]
 struct Args {
- arg_input: Option<String>,
- flag_output: Option<String>,
+ arg_input: Option<String>,
+ flag_flexible: bool,
+ flag_output: Option<String>,
 }
 
 pub fn run(argv: &[&str]) -> CliResult<()> {
  let args: Args = util::get_args(USAGE, argv)?;
  let conf = Config::new(&args.arg_input).no_headers(false);
 
- let mut rdr = conf.reader()?;
- let mut wtr = Config::new(&args.flag_output).writer()?;
+ let mut rdr = conf.flexible(args.flag_flexible).reader()?;
+ // write is always flexible for performance
+ let mut wtr = Config::new(&args.flag_output).flexible(true).writer()?;
  let mut record = csv::ByteRecord::new();
 
  while rdr.read_byte_record(&mut record)? {