Add some code quality tools and CI automation

[egregors]

To make the project more pretty for contributing and easy for PR management we should make a few things:

• add gofmt and golint into Makefile
• add pre-commit hooks with formating code and linting
• add GitHub Action with the install, build, lint, test steps for all PR's
• add GitHub Action for automating upload new version into brew

gofmt – std tool for formatting source code, running it before commits are dramatically good practice.

golint – the same, but for linting. This tool usually helps write code more idiomatically

even now (with really small code base) golint suggest many improvements:

➜  underblog git:(master) ✗ golint ./app/...
app/cmd/blog.go:18:7: exported const DefaultMarkdownPath should have comment or be unexported
app/cmd/blog.go:20:1: comment on exported function NewBlog should be of the form "NewBlog ..."
app/cmd/blog.go:71:13: should replace errors.New(fmt.Sprintf(...)) with fmt.Errorf(...)
app/cmd/blog.go:77:13: should replace errors.New(fmt.Sprintf(...)) with fmt.Errorf(...)
app/cmd/blog.go:130:16: method copyCssToPublicDir should be copyCSSToPublicDir
app/cmd/post.go:17:1: exported function NewPost should have comment or be unexported
app/cmd/post.go:62:6: exported type Post should have comment or be unexported
app/cmd/underblog.go:9:1: exported function MakeBlog should have comment or be unexported
app/cmd/underblog.go:14:10: should replace errors.New(fmt.Sprintf(...)) with fmt.Errorf(...)
app/internal/cli.go:7:6: exported type Opts should have comment or be unexported

@freetonik are the GitHub actions available for this repo? Only owner or collaborator may figure it out