Add vendor path to be ignored

Greetings, I've been using Harp.js from some time ago and I realized that when one wants to compile a project, harp will work with each file it finds inside project folder. This can be a drawback when working with tools like Bower. Which creates a subfolder for components a.k.a ```bower_components```. I've seen the open issue about this, but there is no PR yet.

I figured out that the simplest way to do this is passing the compile command a parameter specifying the location of the vendor folder, by default inside ```[publicPath]/vendor```. Harp will ignore this path when compiling and will copy it without altering it to the ```outputPath.```

I read somewhere that ```.gitignore``` can be used for this purposes, but knowing that harp's approach is simplicity, I think this solution addresses the problem properly, since it decouples the solution from other factors (like git). And I'd say it covers the 80% or 90% percent of cases when someone needs to ignore a folder.

With respect to the code all test are passing and I've followed harp's coding style.

I've already used this for one mobile app we're developing with Phonegap and an API based static web app. So far so good. No errors and now I can use bower with ease.

I'm keen to any commentary,

Néstor Turizo