Updated plugin file writing to use NSString.writeToFile_atomically

[chriscohoat]

The line:

NSString.stringWithString(svgString).writeToFile_atomically_encoding_error(path, true, NSUTF8StringEncoding, nil)

Was giving me an an error. You can reproduce with something like the following:

var errorPtr = MOPointer.alloc().init();
var file = NSString.stringWithString(svgString).writeToFile_atomically_encoding_error(path, true, NSUTF8StringEncoding, nil);
print(str);
print(errorPtr.value())

My changes are the only way I can seem to get the SVG to write to my filesystem. I also added a helper method to save the file along-side the original SVG, which people may want if they want to retain the original version of the SVG.

Sketch version 55 and I'm on macOS Catalina (Version 10.15.3).