Enhancement: Console print a list of available targets

[erikd234]

mage -h gives me the mage tool help

but I really want something like mage -l or mage -list that lists all the available targets and a description for that they do, I have so many targets and I am forgetting the names and have to look in the mage file which gets frustrating.

IF we are open to this i'm happy to contribute a PR let me know :)

also let me know if this feature already exists and I am missing it.