jp_DesignDoc: Rename command name (issue #2)

* snip -> ssnip
* spm  -> sspm
jp_DesignDoc: Indent example code (issue #3)
jp_DesignDoc: Use `bash` instead of `shell` to represent command-line

It is to prevent a lexer error in pygment.
jp_DesignDoc: Change `sspm-publish`'s command-line option (issue #4)

* change `sspm publish {repo-name} {url}` -> `sspm publish {url}`
* repository's name should be defined in `snippet.json`
jp_spec_snippet_json: Add jp_spec_snippet_json.md
jp_spec_tag: Add jp_spec_tag.md