More flexible namespace map

[CJ-Wright]

Currently the namepspace map is loaded from a file. It would be nice to be able to provide it from calling code.

CC: @jkarp314

[ericdill]

Shouldn't we just load it from the conda forge map you and @mariusvniekerk built?

[CJ-Wright]

We could, I thought it would be more robust to allow 3rd parties to provide it if they wanted to.

How do you want to implement that, do you want to pull from the url at runtime?

[mariusvniekerk]

I'd recommend we vend in the current state of that file and pull it on demand

[mariusvniekerk]

And I guess read from some RC file

[CJ-Wright]

xref: #49