Enable more linters

Enables the following linters:
- `deadcode` finds unused code.
- `depguard` checks if package imports are in a list of acceptable
  packages.
- `misspell` finds commonly misspelled English words in comments.
- `nakedret` finds naked returns in functions greater than a specified
  function length.
- `varcheck` finds unused global variables and constants.

Our repo was already passing these linters so no fixes needed.

Part of #217

Signed-off-by: Andrew Seigner <siggy@buoyant.io>

.golangci.yml

@@ -6,9 +6,26 @@ linters:
   enable:
   - gofmt
   - golint
+  - depguard
+  - misspell
+  - nakedret
+  # TODO: enable more linters!
+  # - dupl
+  # - gochecknoglobals
+  # - gochecknoinits
+  # - goconst # TODO
+  # - gocyclo
+  # - goimports
+  # - gosec
+  # - interfacer
+  # - lll
+  # - maligned
+  # - prealloc
+  # - scopelint
+  # - stylecheck
+  # - unconvert
+  # - unparam
   disable:
-  - deadcode
   - errcheck
   - staticcheck
   - structcheck
-  - varcheck