Add .htm suffix to temp files before passing to browser

Hi! I’ve started to use Gadfly and am very impressed with the graphics. However, I had difficulties with getting my web browsers to open suffix-less temp files (This is on a Mac). This patch adds a .htm suffix to the temp files before opening them in the browser. 

Technically, this does not guarantee uniqueness as mktemp does, but I’m not sure what a better solution would be with the current file.il calls.