Release ciigo v0.2.0 (2020-07-05)

* all: simplify serving content using function Serve
  Previously to serve the generated content we call two fucntions:
  NewServer() and Server.Start().
  This changes unexported the internal server, and expose only the
  Serve() function with the same parameter as NewServer().

* all: embed the HTML template and the stylesheet
  The parameter for template either in Convert or Generate functions or
  in CLI now become pure optional, not default to "templates/html.tmpl"
  anymore.
  This will minimize steps for user to setup or run the library or
  program.