[WIP] if csvstack files have headers, union the headers and align data

[fgregg]

This PR will address #245. If the files input to csvstack have headers, then csvstack will create a header that is the union of those headers and align the data to those headers.

Todo

• [ ] handle case where one of the input files is stdin
• [ ] write tests for new functionality

[fgregg]

@jpmckinney , if you'd like to take an initial look and see if this approach is to your liking, that would be great. Feel free to wait until tests (and new test are written too).

In particular, i tried to match the current style of csvstack. If you'd like a more modular approach, lmk.

[jpmckinney]

Looks good from a quick read! If you think a more modular approach would be easier to read and/or maintain, I'm happy to follow that approach.

[fgregg]

@jpmckinney, two years later. i think is is ready for your real review.

[jpmckinney]

Thank you! I added a commit but couldn't push to this branch, so I've opened #1193.

[jpmckinney]

1.1.1 is now released with this feature 🎉