Improvements

• Added attachments management: list, open, insert attachments from file or url (solves #24)
• Notebook list can show stack names and be sorted (#30, thanks @danielfrg)
• Added wiki_tables setting to enable Wiki-style tables syntax (solves #18)
• Now notes in webapp are opened in default browser (solves #22)
• Configuration asks for noteStoreUrl so the plugin works properly for yinxiang.com (solves #20)