Prefer `start_with?` over `starts_with?`

Use parentheses for method call with arguments.
Reduce method length of ReadBoardMenu
Add util methods to ReadBoardMenu

- `selected_completable_board?`
- `selected_readable_board?`
Rename `read_post.rb` to `read_post_helper.rb`

Follow the convention of Rails. There is
`app/helpers/application_helper.rb`.
Update documentation

- `TermApp::ReadPostHelper`
- `TermApp::ReadBoardMenu#scroll`
Keep line length to 80
Add `TermApp::ReadBoardMenu#read_post`
Reduce method length of SelectBoardMenu

Added `TermApp::SelectBoardMenu#matched?`.
Reduce method length of ReadBoardMenu

Added following methods:

- `Board#posts_from`
- `Board#posts_to`
- `TermApp::ReadBoardMenu#sanitize_current_index`
- `TermApp::ReadBoardMenu#print_select_board`
Merge pull request #107 from yous/patch-method-length

Reduce method length