Add compileDependencies option

Redux of last week. This is the same operations, but runs faster and is moved to an outside resource.

I have added the option `compileDepedencies`, when true this adds a partial's dependent files to the current async process. I have added sass-get-dependencies as a dependency on the package.json. Written tests and updated the documentation / Readme file.
1. It's definitely not what this option really does.
It finds the files that depend on the `src` (globbed) partial files and compiles them.
While on that topic... (see 2 below).

2. IMO the option name is misnamed (as well as identifiers in the code and the module name).
The name might be confusing. It should be `compileDependents`, shouldn't it?