Add config option to copy certain files instead of compiling them

[chkal]

I use bower to include dependencies for my site in the public directory. Bootstrap includes the LESS source files which is good, because a common pattern is to not use the compiled Bootstrap CSS but to use the LESS files to created customized versions of Bootstrap.

If I include Bootstrap in the public directory, the compile fails because harp tries to compile each LESS file separately. This doesn't work, as all variables are declared in a separate file, so you get "variable @brand-info is undefined" and similar errors all over the place.

The first commit is a test reproducing this issue.

The second commit defines a new configuration property which allows you to specify glob patterns for files that should be copied instead of compiled. That is basically something that also came up in #275 (see my last comment there).

{
  "copy": [
    "vendor/**",
    "*.less"
  ]
}

I think this concepts could be extended to also offer an "ignore" config property for ignoring files which would provide a feature that has often be requested (see #275, #325, #312 )