Append to output file instead of rewrite it

I have an issue when use both less and [css](https://github.com/guybedford/require-css) plugins in the same project. Both plugins write result to the same file. The last added plugin rewrites results of the first one.

I've replaced rewrite to append (requirejs optimizer clears destination directory - we don't need to worry about previous results). It is better to switch to the same behavior in css plugin.

Looks like this plugin works only with Node (`less` and `path` packages loaded using Node `require`). So, I haven't changes Java's part of `saveFile`. Maybe, it should be removed at all?