cli : generate mnemonic without interactive mode

[qrayven]

Description

Generation of mnemonic without interaction mode
example:

/wallet mnemonic –output-file-name file-name.txt or ./wallet mnemonic –output-stdout.

Motivation

CLI is many times used for simple shell automation scripts.

Requirements

Write a list of what you want this feature to do.

1. ./wallet mnemonic --output-file-name filename.txt will generate a mnemonic to the file
2. ./wallet mnemonic --output-stdout will generate a mnemonic to the stdout

Open questions (optional)

name of flags

Are you planning to do it yourself in a pull request?

Yes/No.