Auto-indexing option

Starting from V0.1.17 Fossa Guard Pro supports the option to auto-index S/MIME emails during reading meaning that the search index is auto-updated.

Emails that are left unread can be indexed manually by clicking `Refresh` link it actualize the index from last update time up to the current time.
Note that the search index is limited by extension local storage size quota around 5Mb https://developer.chrome.com/apps/storage#property-local