Improve documentation and cmdline help

certo/__main__.py

@@ -1,11 +1,14 @@
 """
-Usage: certo [-v] [-j] <hostnames>... [-d DAYS|--days-to-expiration=DAYS] [-t SECONDS|--timeout=SECONDS]
+Usage:
+    certo [-vj] <hostnames>... [-d DAYS|--days-to-expiration=DAYS] [-t SECONDS|--timeout=SECONDS]
+    certo -h | --help
 
 Options:
-   -v                                 Increase verbosity [default: False]
-   -j                                 Output in JSON format [default: False]
-   -d DAYS --days-to-expiration=DAYS  Warn about impending expiration if within DAYS of the cert's notAfter [default: 5]
-   -t SECONDS --timeout=SECONDS       Timeout for SSL Handshake [default: 5]
+   -h --help                          Show this help.
+   -j                                 Output in JSON format [default: False].
+   -v                                 Increase verbosity [default: False].
+   -d DAYS --days-to-expiration=DAYS  Warn about near expiration if within DAYS of the cert's notAfter [default: 5].
+   -t SECONDS --timeout=SECONDS       Timeout for SSL Handshake [default: 5].
 """
 import logging