RSA key serialization

[rca]

I didn't notice a way to serialize an RSAKey object, so I created the following.

Thoughts?

Calling key.toJSON() creates an object that is serializable with:

JSON.stringify(key.toJSON())

The RSA key can be recreated using:

RSA.parse(JSON.parse(rsaString))

[BinaryMuse]

As implemented in this PR, RSAParse already does a JSON.parse, so the example would be:

RSA.parse(rsaString)

Took me a moment to realize this, so perhaps this note will help someone else.

[stephensprinkle-zz]

Fantastic! Thanks so much. Got stuck on this aspect in a project and this solved it.

[hawkps]

This is awesome, really helped me out! why isn't it in the official file?

[kitwalker12]

:+1: This is awesome. helped me out with a case where I wanted to encrypt at one location and decrypt elsewhere without sharing the pass phrase

[simkessy]

Can we pull this in? I'd really like to store the key in my application without sharing the passphrase

[therightstuff]

are @wwwtyro and @rca interested in continuing this? i'm happy to jump in or fork and pull and maintain if not. this is really useful and all that's blocking it is an extra comma...

[beenotung]

it seems merged somehow, this PR can be clossed

[ayazzali]

coool! but its realy hard to find.

[dansgithubuser]

This repo is behind the npm distribution? That's exactly the kind of thing I don't want in an npm package offering security....

edit: ended up using web crypto, I needed symmetric encryption too https://developer.mozilla.org/en-US/docs/Web/API/SubtleCrypto