Use a default datastore path when not provided

[bradlarsen]

Currently you have to explicitly specify the datastore to most Nosey Parker commands. For example, to scan something, you have to say noseyparker scan -d DATASTORE SOME_INPUT, and to report findings, you have to say noseyparker report -d DATASTORE.

It can be a hassle to keep specifying the datastore over and over. Additionally, the -d/--datastore argument must be given after the subcommand name, which complicates command-line editing. For example, to scan and then report, you first run scan, then go to your previous command in shell history to change the subcommand to report, but you have to edit within the line, rather than being able to move the cursor to scan, deleting until end of line, and typing report.

You can kind of work around this hassle at present by setting the NP_DATASTORE=DATASTORE environment variable, which lets you get away with saying just noseyparker scan SOME_INPUT and noseyparker report.

It would be nicer if Nosey Parker would choose a default datastore name if it is not explicitly specified. The proposal is this:

• Use a default value of datastore.np for the datastore
• To avoid a UX hazard of accidentally polluting an existing datastore, modify the scan command so that if no datastore is explicitly given via -d/--datastore or the NP_DATASTORE environment variable — i.e., the default datastore path is used — and the datastore already exists, exit with an error