Allow scanninng only specific location instead of whole workspace

[SharakPL]

Fixes #153

Adds option "scss.scannerRoot": "". Default value is empty string - in this case whole workspace is scanned. It allows to set more specific location where files should be scanned instead of messing up intellisense with duplicated variables from other locations in the workspace. Example:

/your/workspace/.vscode/settings.json

{
  scss.scannerRoot: "themes/my-new-theme/src/scss"
}

This will scan only .scss files in /your/workspace/themes/my-new-theme/src/scss so you can remove all paths from scss.scannerExclude array that are outside scss.scannerRoot location