Create set of task specific tools

Currently we support all the functionality in a monolith binary called 'cfssl'. Smaller, more task-specific tools that are easy to use would be helpful.

Add the following new commands in the /cmd folder:

• scan: A dedicated CLI for "cfssl scan" that pretty-prints the output
• bundle: Allows provides the functionality of bundle, prints the bundled output to stdout and the messages to stderr
• newkey: A simple CLI for creating a key and a CSR. Supports writing to file, reading configuration from either a CSRJSON or if one is not provided, the command prompt.
• certinfo: A replacement for "openssl x509 -text -noout" that uses the certinfo command to extract information from the certificate and pretty-print the information