Improve command line interface

The `into` command #69 provides a command line interface to the `into` function

```
into myfile.csv myfile.json --delimter ;
```

Here is a list of possible improvements
- [ ] Decrease `into` import times and latency
- [ ] Support keywords anywhere, e.g. `into myfile.csv --delimiter ; myfile.json`
- [ ] Some way to discover keywords based on the file type `man into`, `into csv --help`, etc...
- [ ] Cleaner error reporting than the current Python stack traces
- [ ] Progress bar or at least some reassuring token that promises that progress is being made

@cpcloud and @quasiben came up with most of these ideas. Probably I'm forgetting a few. Please treat this comment as wiki space and add appropriately.

Start with the `into` command introduced in issue #69 and inspect its current argument parsing, import path, and error output. Done would require deciding which listed improvements are in scope and implementing the chosen CLI behavior, including keyword placement, discoverability, errors, latency, or progress feedback.

Written by the indexing model from the issue text.