Add `sort` option for sorted strings in output file

Adds a `sort` option (shorthand `-s`) to allow people to get sorted
strings in the resultant file. Sorting makes the `po/pot` files
easier to follow with version control, preventing arbitrary moving of
unchanged strings in the file. It also makes debugging easier.
Merge pull request #32 from BYK/sorted-output

Add `sort` option for sorted strings in output file