handle utf-8 chars

Hi. 

I've made a change to make the content of the file to be concatenated unicoded so that chars like ÅÄÖ wouldn't cause errors.

All I did was add an extra step of unicoding the file content variable (line 46 in concat.py). This solved the issue for me, and it now accept files with special swedish chars in it.

Im not a python dev at all, and I'm no expert on different charsets so maybe this is a bad way of doing things, but it seems to be working as intended, at least for me. But if you know of any pitfall that I'm unaware of, please let me know. Else you might want to merge this change into your superb sublime package!

And thanks for sharing. :)

/J