Add "edit" link to wiki pages

[wincent]

Much like we leverage GitHub to show the edit history, may as well provide edit links too that just link to the appropriate page on GitHub for editing the file and submitting a pull request.

Example URLs:

• https://github.com/wincent/masochist/edit/content/content/wiki/%22HTML%20is%20probably%20what%20you%20want%22.md
• https://github.com/wincent/masochist/edit/content/content/wiki/ANTLR%20lexer%20cheat%20sheet.md
• https://github.com/wincent/masochist/edit/content/content/wiki/Is%20Mercurial%20better%20than%20Git%3F.md