Mechanism for explicitly stating CSV or JSON or TSV for sqlite-utils memory

[simonw]

• Implement filename.json:json and -:nl and suchlike options for specifying the format rather than guessing it - see Idea: import CSV to memory, run SQL, export in a single command #272 (comment)

Follows #272

[simonw]

The -:json idea is flawed: Click thinks that's the syntax for an option called :json.

I'm going to do stdin:json - which means you can't open a file called stdin - but you could use cat stdin | sqlite-utils memory stdin:json ... instead which is an OK workaround.