Add option to write key pair in stdout

[joemarct]

This adds an option to just write the key pair in stdout and not write it out into a file.

This is more convenient if we're calling this from a script in another runtime (e.g. Shell or Python) because we don't have to write to file and read from it just to get the address and private key.

To write only the address and private key (which is what I needed in my use case), you can now run the command as: dcraddrgen -noseed -nofile

[Zergity]

Simpler solution #32