Add support for forced custom implementation

[Protryon]

This adds a feature force-custom that will supersede target_os based rng selection.

This fixes #230, and supports my particular use case, which is building for wasm32-unknown-unknown in theory, but using wasm32-unknown-emscripten in practice, with a custom rand impl.

[newpavlov]

We intentionally did not want an easy way for users to overwrite the "default" sources on supported platforms, since it may cause security issues. Imagine a crate which silently enables the force-custom feature and uses a backdoored RNG.

If users absolutely need to use a different entropy source on supported platforms, then they always can override getrandom with their custom crate.

[josephlr]

I agree with @newpavlov here. We don't want to allow unilateral overriding of the RNG, and there are other ways to achieve your goal here. Note that when we add #275, it may help your use-case, as you could then link against a different getentropy.

I'm going to leave #230 open, as it deals with out desired preference for rdrand vs custom.