support sublime text2 and fix filename input

On Sublime Text2, old code says "Error parsing snippet xml: Error document empty." and doesn't work. So, support sublimeText2 style file.write() function to fix it.

File Name input panel's placeholder is now just "Default", and add  '.sublime-snippet' later. I think this is easier to edit.
fix wrong editing
Merge pull request #3 from tsukimizake/patch-1

sublime text2 support and fix filename input