Add MFA support

[kevdliu]

First attempt at adding MFA support. Overview of changes:

• Added --mfa argument to the CLI which allows the user to provide a MFA code for login.
• Since Abode uniquely identifies clients by a combination of the UUID and a session cookie, I modified the cache to persist session cookies to disk after successfully logging in and loading it back into requests.session during initialization.
• Added some constants and error for MFA
• I had to modify the CLI logic a little bit for when a MFA code is provided. I decided against adding mfa_code as an argument and a property to the Abode class since the code is very time sensitive and there's no guarantee that it'll be used for login immediately in that fashion. If the user provides a MFA code, we'll log in using the login() function first before fetching devices from Abode.

[coveralls]

Coverage increased (+0.2%) to 83.833% when pulling 2fd0aa1 on kevdliu:master into 2bed8dc on MisterWil:master.

[MisterWil]

This looks great! Thank you so much for working on this. I really appreciate the tests too. I left a single comment, and really it's only a suggestion. If you feel like it's working well feel free to mark it ready for review and I'll merge it in and push out a new version.

[kevdliu]

Good call on handling unknown mfa types. I changed the PR to include that and added some related tests. I've been using this for the past week and no problems so far 👍

[MisterWil]

Great! Thank you!