feature: Adapt -a/--all option for archives command

[jcs090218]

You can now use

$ eask archives --all

to list out all archives that are available!

Example output:

...

List all available archives:

  gnu           https://elpa.gnu.org/packages/                    0
  nongnu        https://elpa.nongnu.org/nongnu/                   0
  celpa         https://celpa.conao3.com/packages/                0
  jcs-elpa      https://jcs-emacs.github.io/jcs-elpa/packages/    0
  marmalade     https://marmalade-repo.org/packages/              0
  melpa         https://melpa.org/packages/                       0
  melpa-stable  https://stable.melpa.org/packages/                0
  org           https://orgmode.org/elpa/                         0
  shmelpa       https://shmelpa.commandlinesystems.com/packages/  0

(Total of 9 archives available)

[jcs090218]

Just renaming these variables

• keywords -> keys
• keyword -> key

[jcs090218]

Add test.