Disambiguate `data get [dataset]` and `data get` from Datafile

@jbenet,

What are your thoughts on disambiguating (1) fetching a single dataset from (2) fetching a collection from a Datafile? Is this something you've already considered and made a decision to avoid?

Proposing the following api modification:

```
$ data install
# downloads datasets given Datafile
```

This would pave the way for users to explicitly specify a Datafile without excessively overloading the `get` command.

```
$ data install [-arg to specify datafile] Datafile.staging
```