feat: migrate from docopt to cobra for CLI argument parsing

Replace docopt-go with spf13/cobra, giving cheat a built-in
`--completion` flag that dynamically generates shell completions for
bash, zsh, fish, and powershell. This replaces the manually-maintained
static completion scripts and resolves the root cause of multiple
completion issues (#768, #705, #633, #632, #476).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

vendor/github.com/spf13/pflag/.travis.yml

@@ -0,0 +1,22 @@
+sudo: false
+
+language: go
+
+go:
+  - 1.9.x
+  - 1.10.x
+  - 1.11.x
+  - tip
+
+matrix:
+  allow_failures:
+    - go: tip
+
+install:
+  - go get golang.org/x/lint/golint
+  - export PATH=$GOPATH/bin:$PATH
+  - go install ./...
+
+script:
+  - verify/all.sh -v
+  - go test ./...