Allow N, K, and H to be specified per command.

[PRabahy]

It would be nice if there were more places to specify N, K, and H than just the tahoe.cfg file. For example, if I have a very important file that I'm uploading, I might increase K or H. On the other hand, if I'm uploading something big like a movie, I might turn up N for that file to make the disk usage smaller.

I could also see some benefit of having these recorded per alias in the .tahoe\private\aliases file. For any upload action to a specific alias, I would expect the alias definition to override tahoe.cfg.

[PRabahy]

Somewhat related to #84 and #1252.

[dawuud]

+1!

I agree the aliases file would be a convenient place to store this information... although that does lead me to remark that then we'd have structured data inside a text file which the user is responsible for editing; therefore we should replace the aliases file with aliases.yaml

What would the commandline options look like? I guess the new options should apply to the tahoe put, cp and backup commands and if unspecified default to the N,K,H as specified in the tahoe.cfg