Add a `show` command

[akaoj]

So that you can check a key (and certificate) characteristics without having to know the OpenSSL syntax. Something like:

$ simpleca show client --name www.domain.com
Please provide the password for the file clients/www.domain.com.key:
Key www.domain.com is valid:
  Type: ECDSA 256 bits
  Encrypted: yes
Certificate:
  Signed by: intermediate01
  Valid from 2018-01-30 until 2018-09-30
  Serial number: 903874820725074085822
  SHA256: 63f9339e96d31f926c58739f14bacbadbbf38576fa93ba0eadba4a0c509a8d8b

[akaoj]

How will we handle keys we don't have (see issue #4)? We can't display information about them...