Added config file support to specify paths of LESS files to compile.

Added functionality to read from a compilation configuration file that allows the user to specify the paths of less files to compile.  If the config file is not found then the extension operates as normal.  This addition is particularly useful when several LESS files import a common file and those LESS files need to be updated whenever that file changes.

As of right now it is only looking for a file name "compile.json" in the root of the project, but this can be incorporated with extension preferences so the user can choose another file.  In the future, this file can house other file types like JS for other minification/compilation tools.  

One issue is that this config file is read every time LESS is compiled, and only needs to be read when the file is updated, but this works fine as a quick addition.

### Example "compile.json"
```json
{
    "less" : [
        "path/to/less/file.less",
        "another/less/file.less"
    ]
}
```