No deletion of options.remotePath

Hello Sindre,

The 'remotePath' option is currently deleted when gulp-ftp runs for the first time : https://github.com/sindresorhus/gulp-ftp/blob/master/index.js#L16 .

If 'options' is a reference to a previously defined object, this deletes the original definition of 'remotePath' and prevents this object from being used again, like with 'watch'.

Example: https://github.com/vkammerer/mekaviz.com/blob/master/gulpfile.js

Not deleting the property, and passing the options to JSFtp as in this PR solves the problem.