Fill empty values subcommand

[alexrudy]

This adds a fill subcommand. I'm not strongly opinionated about this being in xsv, but I find it useful, so I thought I would make the PR and see.

Motivation: I have CSVs (usually from DB dumps) with empty values in certain columns, which I want to fill forwards or backwards using the next valid value.

Implementation: Iterate through rows, gather and store the last valid value, and fill in missing values as they are found. The --backfill argument fills empty values which occur before the first missing value, and so must buffer.

There is also a --groupby feature, something that XSV doesn't do very often, but which I find helpful in this case, to group by e.g. (in application logs) user_id, if the filled value should only apply to each user, or by app_version if the value really applies to each app version. This does increase the memory overhead by the order of the number of groups.