Lua scripting support and 'script newcolumn' command

[Dimagog]

'xsv script newcolumn' command allows creation of new columns,
calculated by Lua script based on current and previous rows.

Many other commands can be added in the future
(map, filter and reduce come to mind first),
that's why there is an extra 'newcolumn' sub-command.

[Dimagog]

I'm working on unit tests but decided to publish it early for code review.

[Dimagog]

Tests are done and I've merged with the latest master. Ready for review.

[Dimagog]

@BurntSushi do you have any comments/concerns for this change?

[BurntSushi]

@Dimagog I think this looks like a potentially neat change, but I'm not sure if it's a good fit. Practically speaking, adding a C dependency increases maintenance burden. I have very little time for this project as it is, and I really do not want to deal with a dependency on an entire programming language.

In general, I'm not sure if it's a good fit for xsv, which is focused on implementing common operations very quickly.

My suggestion is that if you want a CSV tool that permits arbitrary scripting, then it might be better to go and build that tool separate from xsv.

[Dimagog]

No problem, I’ll have to keep the fork then.
For my needs I had to use 3 separate tools at first, but wanted to converge on just one. So I’ve picked xsv (partially because I wanted to play with Rust :-) ) and added missing functionality. Now that it does what I need I have no plans to evolve it.