Incompatibility warnings

[martinheidegger]

Using sodium-universal is for people that don't want to concern themselves with platform incompatibilities (one system to work with all). As such I think it would be good for developers to have a means of using sodium-universal that also highlights if they are using something that doesn't work for some platforms. Something like:

const sodium = require('sodium-universal')
sodium.crypto_stream_xchacha20(/*...*/)

will run following in console:

[WARNING] crypto_stream_xchacha20 is not supported in browser versions.
  → you can disable this message by using: require('sodium-universal/unsafe')({ crypto_stream_xchacha20: true })
  → issue 1a: https://github.com/sodium-friends/sodium-javascript/issues/#1a

This way a person could easily see what the progress is and that the application they are building is currently not as compatible as they may think it is.

In a future step I could imagine that in a future release that supports crypto_stream_xchacha20 it could warn the other way around:

[INFO] crypto_stream_xchacha20 is now supported on all platforms since version x.9.9
  → you can disable this message by removing the `crypto_stream_xchacha20` property
    when in your require statement: require('sodium-universal/unsafe')({ ... })

I think it would be reasonable to add this to the library (including all the issues on github). Would you be okay with PR's that add this?

[RangerMauve]

Maybe it should be under a DEBUG flag in case folks don't want extra console output?

[mafintosh]

Easiest/best way would prob be to add a more where it force uses the JS bindings so you can see what breaks.