Add config file for specifying options

[jeromekelleher]

Since #35 we can specify the tree sequence file on the command line. This works fine for now, but we'll want to be able to specify a bunch of things to tweak at some point, and this is a good way of making things configurable without having a million buttons on the UI.

I think the standard approach here is for people to also allow overriding config file settings via environment variables.

It's probably worth reviewing the bokeh serve docs and other examples out there for best practises on how to do this. (See also #34)

[jeromekelleher]

This isn't a priority now though - we can just assume that we will have some way of configuring various UI things like the threshold for the number of points to start displaying points at.