Extract ExternalCommand class

[chrismytton]

This adds a new ExternalCommand class which encapsulates calling an external command and checking its exit status. This means that the code for running external commands is now nicely tested and easier to reason about.

Fixes #45
Part of #62

Notes to merger

Please complete the following before merging:

• Change the target branch to master once Do a shallow clone of everypolitician-data repo #71 has been merged.

[tmtmtmtm]

This is very old now, and I can't remember any of the context for why it got left hanging. There have been other substantial changes to the code in the meantime, which means this would need to be rebased over master if we continue with it, but the underlying principle seems good, so it's probably worth picking up again.

[tmtmtmtm]

I got tripped up by this being named as if it were a pure command method (i.e. would have no return value). I think it would be better to name this result or equivalent to highlight that you're actually getting something back from it.

[tmtmtmtm]

it's a bit of a red flag for one of the constructor parameters to be public, and for there to be no behaviour whatsoever in this class related to it.

As the only method is really only a very simple delegation to the other constructor, I wonder if this really needs to be a full class at all, or whether a Struct might be enough for now. Or, perhaps this is a sign that this class should really be taking care of things like cleaning up the output, rather than returning it as-is for the caller to clean up?