Proposal: `noms use`

[aboodman]

For a few reasons, I don't end up using noms config that much:

• I can never remember the syntax and have to look it up each time
• I don't really need the dataset or aliasing features frequently, so their presence is conceptual overhead I have to digest every time
• I can't remember where the file is supposed to go
• There's no CLI support to just do it for me, which is where we put everything else

I think I'd like to replace noms config with noms use. It would work like this:

• Introduce an environment variable $NOMS_DATABASE which is just a database spec. If present, any spec on the CLI that includes a database component can omit it and it will default to this value.
• As a convenience and to aid discovery, introduce noms use which manipulates this environment variable as a convenience

Here a transcript of a sample usage:

# noms use /tmp/mydb
Set environment variable $NOMS_DATABASE=/tmp/mydb

# noms use
Currently using $NOMS_DATABASE=/tmp/mydb
To stop using: `noms use --stop`, or remove the environment variable

[aboodman]

I forgot we already had this discussion:

#1802 and #2131.

Hm. I still find myself reaching for noms use.

[aboodman]

OK, reminding myself how config works, and the debate we had, I think maybe what I'm looking for is just:

# noms use /tmp/whatever
Wrote changes to ~/.nomsconfig
Default database now /tmp/whatever (previously /tmp/old-db)

# noms use
Currently using /tmp/whatever (see ~/.nomsconfig)
Stop using with `noms use --stop`

IOW, store in config file, not env variable.

Separately cli support for setting aliases would be cool.

[aboodman]

I also have a bad feeling that the CLI is not consistent in using config, so there'd have to be an audit to make it consistent.