Implement write_tsv

[hadley]

Should be simple generalisation of write_csv. Use same strategy to only quote when needed.

[npjc]

@hadley I've been reading through your r-pkgs book book and I'm learning me how to package. It's very helpful! Anyway since I wanted to walkthrough the process with a simple use case:

i decided to make writer where I (stole! and) generalised readr:::stream_csv() to stream_delim() and used it to implement:

• write_csv()

• write_tsv()

• write_delim()

  I also threw in naive wrappers around base::saveRDS to make:

• write_rds()

• write_compressed_rds()

Now although this was purely an exercise to get learning.. it got me wondering: If readr is about reading then should you have a writer (or maybe saver since it has 5 letters) package? And if readr is really about stable/streaming data input and output should it then be renamed to reflect this? I guess I'm really asking about the phylosophy of how package-happy one should be when developing thingamabobs.

I hope it's helpful with this issue!
If you have any thoughts rep(c('thanks', '!'),1E6)

[hadley]

I'd rather just keep all the functionality in readr - I don't think it's too awful to keep the name as readr, since that is the primary purpose. Do you want to do a pull request?

[npjc]

@hadley okee: see #157