Add syntax option for syntax highlight
Add syntax option for syntax highlight

usege:
```
$ cat lib/nidobata/version.rb | nidobata post my-org-slug my-room --syntax ruby
```

If you specify the syntax for the pre option,
the behavior will change in the markdown file containing triple backquote(```),
so I added a new option.

idobata/nidobata,idobata/nidobata