URL-Encoding for anchor link inside toc

[toshimaru]

closes #95, closes #101

Description

BEFORE

href inside toc: ```html

あ ``` href inside the anchor: ```html ...あ ```

AFTER

href inside toc: ```html

あ ``` href inside the anchor: ```html ...あ ```

Changes

• CGI.escape for URL-encoding
• Refactoring: Create Helper module
• Refactoring: Do not use Nokogiri::HTML(parser.toc), use parser.build_toc instead