Enhance template customize (devide and allow missing some template files)

This pull request enables the followings to enhance customizability.

* Allow you to copy and modify template file(s) what you want to customize
    * Currently, you must copy all template files of lib/prmd/template/*.erb even if you want to modify only schemata.erb
* Devide schemata.erb into some parts since it is mixture of logic and presentation
    * The current schemata.erb makes difficult to follow prmd updates