Add Environment variables

[jqnatividad]

• QSV_NO_HEADERS (in addition to the existing QSV_TOGGLE_HEADERS)
• QSV_DELIMITER
• QSV_FLEXIBLE (https://docs.rs/csv/1.1.6/csv/struct.ReaderBuilder.html#method.flexible)
• QSV_REGEX_UNICODE

Note: implementation of each env var will be tracked by a separate issue and checked off here as they are done.

[jqnatividad]

Also add QSV_REGEX_UNICODE, as regex unicode support if off by default for search, searchset & replace for performance reasons.

[peterjc]

I strongly prefer TSV over CSV, so a big +1 on adding QSV_DELIMITER which I would set to tab, and avoid having to keep adding -d "\t" on piped inputs and using qsv fmt -t "\t" on my outputs.

Cross reference: BurntSushi/xsv#44

[jqnatividad]

Not doing QSV_FLEXIBLE.

Users are meant to use fixlengths - which works in flexible mode, to fix CSV files, which can then be used with other qsv commands.

[jqnatividad]

Done. Logging, if implemented, will NOT use an environment variable. It will just check the log4rs yaml file.