Use current working directory if path is not set

[karlhorky]

Using the example in the readme below, the file configuration key is not set.

var sass = require('node-sass');
var tildeImporter = require('node-sass-tilde-importer');

var result = sass.renderSync({
  data: scss_content,
  importer: tildeImporter
});

This causes source to be set to the value "stdin", causing the importer to fail.

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This PR reassigns the source to the current working directory of the process if it is set to "stdin".