dont open stdin by default

this makes a breaking change I think

me and @mafintosh were trying to figure out why this was hanging:

```
bionode ncbi search genome eukaryota | dat import --json
```

and it was because of how bionode was always doing `process.stdin.pipe(cli.stdin)`. 

the more unixy way to do it would be to require a `-` as the last argument, which is what this implements, so now if you want bionode to read from stdin you can do:

```
bionode ncbi search genome eukaryota
```

otherwise it won't open stdin