csvgrep: Add case-insensitive option (instead of using flag in regex string)

[executionByFork]

grep has a flag for case sensitivity, I was surprised to find a similar flag does not exist when looking through csvgrep help page.

Issue #248 was brought up a long time ago and closed. While it seems regex is a workaround, a case sensitivity toggle was never added and it would be nice to have one for string searches.

[jpmckinney]

The case-insensitive search example is on the documentation page: https://csvkit.readthedocs.io/en/latest/scripts/csvgrep.html

That said an option would be easy to add.