Add :searchable and :search_help_text to cell properties.

Two new optional cell attributes:

1. `:searchable` - if set to `false`, the cell will **never** show up in search results. `true` essentially has the same effect as not providing the attribute at all.
2. `:search_help_text` - if set will also count towards the cell showing up in the search results in addition to the title. This gives the developer a way to specify cells having other text that can be searched for but not necessarily displayed.