Enable Grunt Templates in Src Parameter

*My first open source contribution ever so forgive me if I'm not doing this right.*


I tried passing a grunt template variable to src paramater and got this error: 
**"<%= path %> is not an existing location"**. 

Also tried using `grunt.template.process()` but that didn't work either.

I think being able to process template variables would help with writing cleaner code. I'd set the ftp build path in a variable and every plugin and task uses the variable either directly or in the worst case scenario I use grunt.template.process().

**I looked in** `tasks/ftp-deploy.js`, **spotted the function that processes the source path on line 32 and figured it's possible to build in** `grunt.template.process()` **before the** `startDir` **check on line 45 to enable template processing without breaking backwards compatibility.** 

Hope this is a useful pull request.

*Bear in mind I'm really new to open source, so this might be a completely naive attempt at adding value to your package.*