Allow columns to be selected using regular expressions

[sd2k]

This allows a selector such as r'a[bc]?' to be used, which would
select columns 'a', 'ab', 'ac' but not 'ad'. This can be used in
the 'xsv select' command as well as anywhere that the '--select' flag
appears.

The interface is different to the one discussed in #155 because I couldn't think of an easy way to include the regex flag when deserializeing the SelectColumns. It matches the regular expression string syntax from Python at the minute (i.e. prefixing a string with r), but it's a little ugly on the shell due to the need for two sets of strings :(

Here's some examples:

 ➜  xsv git:(155-select-column...) cat test.csv
a,ab,ac,ad,be,bf,bg
0,1,2,3,4,5,6
 ➜  xsv git:(155-select-column...) target/debug/xsv select r"'a[bc]'" test.csv
ab,ac
1,2
 ➜  xsv git:(155-select-column...) target/debug/xsv frequency -s "r'a[bc]?$'"  test.csv
field,value,count
a,0,1
ab,1,1
ac,2,1

[sd2k]

BTW I'm not super happy with the user interface here - would happily receive tips on how to implement regex matching as a flag.

[mintyplanet]

I think detecting the regex syntax instead of a flag is a reasonable approach, rather than introducing a new flag for each of the subcommands that use --select.

May I suggest the regex syntax used in awk,sed? i.e. xsv frequency -s '/a[bc]?$/' test.csv
I think that would avoid the need for nested quotes.

[sd2k]

@mintyplanet Yes, I think I prefer that syntax too. I've pushed a new commit switching to that instead.

[CMCDragonkai]

I'd like to extend this idea to having replacement capability. Basically a sort of sed but for CSVs. I currently require the ability to select a particular column and apply a regex replace to each column value. I'm sure this could be extended to this.

[mintyplanet]

We'll want to update the usage text in src/cmd/select.rs to describe regex selectors.

[sd2k]

Thanks for the review!

[mintyplanet]

looks good to me!

[mintyplanet]

Please update the usage documentation in src/cmd/select.rs to describe regex selectors.

[mintyplanet]

@CMCDragonkai Your idea seems sufficiently different from this PR that I would recommend you create a new Issue. If you could provide an example with inputs, outputs and hypothetical xsv command/flag, that would be helpful.

[sd2k]

Let me know if you'd like this rebasing 👍

[sd2k]

@mintyplanet @BurntSushi is this mergeable, in your eyes? If not, what changes would you like to see first? Thanks!

[mintyplanet]

Looks good to me, but I don't have commit rights.
The PR will need to be approved by @BurntSushi

[matt-morris]

This is fantastic -- just what I was looking for. Is there anything we can do to help get this merged in?