Stream support in CLI

[jeromekelleher]

#599 added support for working with streams to the C code. We should also be able to use this at the CLI. See also #657.

We should be able to do this reasonably smoothly, I think. For example, currently

tskit info <file>

requires a single file argument. We could change this to read from stdin if the file isn't specified. I don't think this is breaking backwards compatibility, so I'm happy making this change on a minor version bump.

[jeromekelleher]

This is sort if "nice to have" for 0.3.3, but can easily be shoved down the road.

[benjeffery]

Closing for inactivity and labelling "future", please re-open if you plan to work on this.