Add ability to search by hashtags

As per request here:

https://groups.google.com/forum/#!topic/chocolateygui/0RNP2IaDUDY

ChocolateyGUI should have the ability to search for hashtags which have been assigned to a particular package on Chocolatey.org

Start by reviewing the linked Google Groups request and locating the existing package-search flow in ChocolateyGUI. Determine how assigned hashtags are exposed by Chocolatey.org and define done as returning packages that match a searched hashtag. The issue names no files or tests, so the relevant entry points must first be identified.

Written by the indexing model from the issue text.