Allow prefixing memoized variables with underscores

[teoljungberg]

A convention to avoid instance variables leaking private details out of
their current scope is to prefix the memoized variable with an
underscore. This is commonly used in objects that exposes its values
using instance variables, i.e in how a Rails controller communicates
with its corresponding view.

This commit allows prefixing memoized instance variables to use this
syntax and not generate any warnings.

[Drenmi]

Thanks @teoljungberg. It looks like #5844 might offer the same functionality. Could you have a look?

[teoljungberg]

You are absolutely right, I forgot to look at existing PRs and issues before opening mine.

Closing.