Replace external shell script calls with internal Makefile commands

[atc0005]

Verbatim from atc0005/send2teams#41:

As I've begun doing with my other projects, I'd like to replace content like this:

linting:
	@echo "Calling wrapper script: $(LINTINGCMD)"
	@$(LINTINGCMD)
	@echo "Finished running linting checks"

with this:

.PHONY: linting
## linting: runs common linting checks
# https://stackoverflow.com/a/42510278/903870
linting:
	@echo "Running linting tools ..."

	@echo "Running gofmt ..."

	@test -z $(shell gofmt -l -e .) || (echo "WARNING: gofmt linting errors found" \
		&& gofmt -l -e -d . \
		&& exit 1 )

	@echo "Running go vet ..."
	@go vet ./...

	@echo "Running golint ..."
	@golint -set_exit_status ./...

	@echo "Running golangci-lint ..."
	@golangci-lint run \
		-E goimports \
		-E gosec \
		-E stylecheck \
		-E goconst \
		-E depguard \
		-E prealloc \
		-E misspell \
		-E maligned \
		-E dupl \
		-E unconvert \
		-E golint \
		-E gocritic

	@echo "Running staticcheck ..."
	@staticcheck ./...

	@echo "Finished running linting checks"

It's more verbose (though the golantci-lint options will be moved to an external config file before long), but hopefully more consolidated and clearer what is going on.