feat!: remove external dependencies

Remove the kingpin external dependency, and instead just use Go's stdlib
flag package.

BREAKING CHANGE: Long versions of command line flags are no longer supported.

Makefile

@@ -4,11 +4,10 @@ VERSION ?= $(shell cat VERSION)
 SOURCES = $(shell find . -name '*.go' -o -name 'Makefile')
 
 $(BINARY): $(SOURCES)
-	CGO_ENABLED=0 go build -a -o ${BINARY} -ldflags \ "\
+	CGO_ENABLED=0 go build -o ${BINARY} -ldflags \ "\
 		-s -w \
 		-X main.version=${VERSION} \
-		-X main.commit=$(shell git show --format="%h" --no-patch) \
-		-X main.date=$(shell date +%Y-%m-%dT%T%z)"
+		-X main.commit=$(shell git show --format="%h" --no-patch)"
 
 .PHONY: build
 build: $(BINARY)