Adding search

This PR adds 'Search' functionality (#36) to the app and addresses the Load More issue, where the posts cannot be loaded more than once (#64). The second was fixed implicitly, as it was needed by the search implementation.

I'd love some feedback and review and hope you like it.
I know, that some of the code styles are not in sync with the current style, but I guess this will be fixed, if the other PR gets merged, or if we settle on a style. If it is not ok, please let me know, what I can and should fix.

The Search uses the new API used by HN (https://hn.algolia.io/api).