feat: add generate peer id command

[driftluo]

add generate peer id command

$ ./target/debug/ckb peer-id gen --secret_path ./a.txt
$ ./target/debug/ckb peer-id from-secret --secret-path ./a.txt
peer_id: QmYKM9CJhSsn8kBoAmUGdKiS9kbnJA2f9TXohgNBPKxogB

[quake]

I prefer to use stdin/stdout for these helper functions:

./ckb peer-id gen > key
cat key | ./ckb peer-id show

[yangby-cryptape]

I prefer to use stdin/stdout for these helper functions:

./ckb peer-id gen > key
cat key | ./ckb peer-id show

When the output is binary, usually the default method is output to a file, for example, gzip, bzip2, ssh-key and so on. It's a protection behavior.
Otherwise, after people try ./ckb peer-id gen before they read the manual, they have to reset their terminal. For some terminal tools, they have to restart their terminal tools (both tmux and screen has bug when stdout is binary data). They may lost some data which on terminal before run ./ckb peer-id gen.

[driftluo]

bors r=quake,yangby-cryptape

[bors]

Build succeeded:

• continuous-integration/travis-ci/push