Implemented Apple Keychain support with migration procedure

[aik099]

Implementation details:

• using dedicated alfred-totp.keychain keychain (created on the fly as needed)
• using security command to manage keychain, which results in the following behavior:
  • user is asked for a strong password at keychain creation time
  • the Alfred workflow isn't aware of the that password
  • the keychain itself auto-locks after a certain period of time, and if the user needs to reenter his password to unlock and use it
• using security dump-keychain command to get all accounts with their passwords in plain text for performance reasons
• supports manual account/secret migration from the "~/.gauth" plain text file into encrypted Apple Keychain using gauth add migrate command
• extracted the current config file-based implementation into a separate storage class
• added new Apple keychain-based implementation as a separate storage class
• not configurable, but could be an easy switch between different storage classes
• removed obsolete code for key, hexkey handling in the "~/.gauth", because gauth add only populates secret key

Test Plan

1. backup your existing workflow
2. install the workflow from this PR
3. activate Alfred (Option+Space)
4. type gauth (keyword to activate this workflow)
5. if you're testing this workflow for the first time, then:
   • Alfred window will be closed
   • Apple Keychain with alfred-totp name creation dialog will appear
   • type a new strong password in both fields
   • hit the OK button
   • activate Alfred (Option+Space)
6. confirm that you're not seeing any of the accounts
7. select the Add a new secret entry and hit ENTER
8. you'll be presented with gauth add prompt
9. type migrate (so that the final text would say gauth add migrate) and hit ENTER
10. you'll get a notification from Alfred telling about account migration results from the ~/.gauth file
11. activate Alfred (Option+Space)
12. replace gauth add migrate prompt with gauth
13. confirm, that you see all your accounts

TODO:

• Write documentation.
• Create a smoother migration experience, e.g. selecting the error entry about an empty keychain would automatically migrate the data.

Other

• Will likely replace WIP: Using OSX Keychain as backend #11.
• Closes Store secrets in the Keychain #41

[aik099]

Removed branches of logic that aren't used:

• only the hexkey argument given;
• only the key argument given.

[aik099]

Removed unused code for dealing with key and hexkey configuration variables.