Adding the `xsv replace` command

[Yomguithereal]

Hello @BurntSushi,

A PR adding a replace command to xsv and which is quite similar to search but that will replace all the occurrences of the found pattern across the selected column values. I chose to split it into a separate command instead of adding a flag to the search command not to muddle the tool's semantics and to avoid conflicts with the --invert-match for instance.

Have a good day,

[jwickers]

That would be quite useful, I'm having a bunch of scripts where I have to pipe xsv to sed to cleanup data. Is there a plan to merge a similar feature at some point?