Add get command to access a secret for a key

[mattczech]

This adds a chimney get command which outputs the secret for a given key. This enables chimney to be used within scripts as an alternative to the runtime usage of the Keys.swift file.

Usage: chimney get <key> [options]

Outputs a secret from the keychain for the key

Options:
  -h, --help            Show help information
  -s, --spec <value>    The path to the key spec file. Defaults to chimney.yml

[yammada]

Approved!