Feature request: add environment variables and dotenv file support

[jqnatividad]

so folks can set options like --delimiter using environment variables (CSVLENS_DELIMITER?) and those preferences can be persisted with a .env file.

[pataquets]

Alternatively, since this is meant to configure user preferences, it can be also achieved with a .csvlensrc dotfile, as it is a widely used de facto standard. This would prevent variable sprawl, with perhaps the possible exception of a CSVLENS_RC-type kind of env var to point to a nonstandard .csvlensrc file location other than the customary $XDG_CONFIG_DIR/.csvlensrc, with fallback to $HOME/.config/.csvlensrc.