Move explain to standalone task.

[jeremyjh]

resolves #173

[asummers]

I'm always a bit weary of String.to_atom/1. Should be fine here, though, I think.

[jeremyjh]

Yeah its the best choice here, we can't leak.

[asummers]

explain/1 will always return a truthy value, so this check is really

if module do
  module.explain()
else
  "Unknown warning named: #{warning_name}"
end

Also typo on Unknown

[asummers]

Or if you REALLY want to get fancy:

case Map.get(Dialyxir.Warnings.warnings(), warning) do
  nil -> 
    "Unknown warning named: #{warning_name}"
  module ->
    module.explain()
end

[jeremyjh]

If the warning name isn't found in the map, module will be nil so the left branch will fail and it prints the unknown. Do you just mean the if/case is more readable?

[asummers]

Yeah I think it's a little more honest as to intent. They're all functionally the same.

[asummers]

After comments and tests pass 😄

[asummers]

Might be worth mentioning the new task in the README too