Change init to not blow away existing keys (if exist)

[jstuczyn]

Instead introduce extra init flag like --purge-existing-keys that would force fresh keygen.

[futurechimp]

Thinking about this a bit more, I would even say we should never provide the option to use init to delete keys. We should just force users to rm their existing keys manually so that we know they really really mean it. Simpler for us and nobody ever uses the nym-client to destroy their valuable keys (that's a DIY operation).