added support for a new sass-files option

[randyaa]

Projects that have a large number of sass files but only a few that need to be compiled because of @imports can have a hard time with the CLI because of the automatic compilation included in the current implementation of the sass plugin.

This pull request adds the capability to provide a list of 'sass-files' that restrict the compiler to only compile files that end with the given name(s) or extensions. ends-with was the chosen solution so we can support the default configuration of compiling everything that ends with .scss or .sass

In use this looks like:

  return new Angular2App(defaults, {
    vendorNpmFiles: [],
    sassCompiler:{
      "sass-files":["main.scss"]
    }
  });

addresses Issue 831

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[randyaa]

I signed it!

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CLAs look good, thanks!

[randyaa]

This is already handled by way of the cacheExclude parameter.

as an example, using this configuration in the Angular2App options will skip any files prefixed with a _

    sassCompiler:{
        "cacheExclude": [/\/_[^\/]+$/]
    }

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