test: show code coverage numbers per function

We have added a visualization of code coverage in Coveralls, but
sometimes it is convenient to show percents of covered statements
locally without pushing to remote branch and running CI.

Example of output:

$ go tool cover -func=coverage.out

github.com/tarantool/go-tarantool/auth.go:8:           scramble       90.9%
github.com/tarantool/go-tarantool/auth.go:36:          xor            100.0%
github.com/tarantool/go-tarantool/client_tools.go:13:  EncodeMsgpack  100.0%
github.com/tarantool/go-tarantool/client_tools.go:25:  EncodeMsgpack  100.0%
github.com/tarantool/go-tarantool/client_tools.go:37:  EncodeMsgpack  0.0%
github.com/tarantool/go-tarantool/client_tools.go:49:  EncodeMsgpack  0.0%
github.com/tarantool/go-tarantool/client_tools.go:63:  EncodeMsgpack  0.0%
github.com/tarantool/go-tarantool/client_tools.go:78:  EncodeMsgpack  0.0%
github.com/tarantool/go-tarantool/connection.go:67:    Report         0.0%
...

Author: ligurio

Makefile

@@ -63,6 +63,7 @@ coverage:
 	go clean -testcache
 	go get golang.org/x/tools/cmd/cover
 	go test ./... -v -p 1 -covermode=atomic -coverprofile=$(COVERAGE_FILE) -coverpkg=./...
+	go tool cover -func=$(COVERAGE_FILE)
 
 .PHONY: coveralls
 coveralls: coverage