Discard an extra blank line in the end of buffer generated by jsfmt.

When jsfmt formats a file and print the result to the stdout, it puts a blank line to the end of the formatted source code.
So, vim-jsfmt also appends a blank line to the end of buffer whenever `:JSFmt` is executed.

To avoid this, I added a code to remove the blank line after jsfmt executed.

Alternative solutions as as follow:
- Apply `jsfmt -w` instead of `jsfmt` to the temporary copy and copy it to the buffer.
- Send a patch to jsfmt and fix to avoid the blank line generation.