Squashed 'zsh/antigen/' content from commit 8eabe7c

git-subtree-dir: zsh/antigen
git-subtree-split: 8eabe7c7d5ee12437673bd48a9fd9f35981828c4
Merge commit 'ed38a7366359dbe63c8f49d7f2920dd46b5e1d53' as 'zsh/antigen'
Add antigen zsh plugin manager