add new email feature

[caberos]

#1482

• slcli email list
• slcli email detail
• slcli email edit

[allmightyspiff]

A good start, thanks.

• Change the CLI path to be slcli email list since this function lists out all accounts.
• Update some of the function names to match the style of the rest of the slcli
• Make the Stats/Overview tables into long column tables, not key value tables to look more compact
• your get_statistics method should set some default values if none were supplied.

[ATGE]

Testing

• ✔️slcli email -h
• ✔️slcli email list
  Looks good,

$ slcli -h | grep email
  email

just shows the command email, adds a description and will look better.
something like:

$ slcli -h | grep email
  email Manage Email Delivery Service

you can add the descriptioon by adding:
"""Manage Email Delivery Service.""" in the SoftLayer/CLI/email/__init__.py file

• add email in docs/cli.rst too

[allmightyspiff]

Fix the docblock in email/list.py and @ATGE's comments, and this should be good to go.

Thanks.