Add customisable userSettings

[kwakwaversal]

userSettings can now be passed in on initialization via
helper.init(runtimepath, userSettings) as well as during the runtime
using helper.settings(userSettings).

This makes it easier for unit tests to emulate their production
environment.

Fixes #21

[knolleary]

Thanks for this - sorry for the delay in reviewing, but all looks good.

👍 🚀

[cdlans]

Thanks for this feature! However this implementation has the same issue #35 as my PR #23 had...

[knolleary]

@cdlans my apologies. I forgot about #23 and had a specific request to make settings available and this PR popped out.

[cdlans]

No problem. I am happy the feature is implemented now. But do you think the issue #35 could be solved? Maybe by cloning the userSettings parameter before modifying it? If not, then I think it should be documented somewhere that the parameter is modified and cannot be reused.

[cdlans]

@knolleary do you think cloning the userSettings before modifying it is a good idea? I would be happy to issue a PR.