Be able to search within a keyword or category

[carols10cents]

Great idea from @wesleywiser on #488:

It seems like the libraries::ffi::C category will have 80% of the crates and the other categories will be extremely sparse by comparison. It also seems to me that if I want to use an external library, I already know the name of that library so what I really want is a way to search by library name and filter to just FFI crates.

I guess it just seems strange to me to have a separate category for such a small number of crates. libraries::data-structures, for example, might have hundreds or even thousands of crates in it. Breaking up the "language ffi crates" into sub categories seems like a case of "premature categorization" if you will.

Sorry to keep bringing up search filters for categories, but if you could limit search results by category, then searching for "ruby" within libraries::ffi seems like it would be sufficient.

[carols10cents]

Changed my mind about this being easy. The problem is that each query parameter to the crate index API route is currently mutually exclusive, so this code would need to be rearranged to allow for building up the SQL to possibly include BOTH the full text search AND the category or keyword conditions.

Then we'll need to decide whether to add a different search box that's closer to the category/keyword heading, or just change the behavior and appearance of the search box at the top to make it clear that using it on a keyword or category page will search within that keyword or category and not the whole page.

This could be done in pieces-- just getting the API accepting multiple parameters and using all of them could be one PR, and the UI stuff could be another.

[carols10cents]

The backend part is done - see #666! Now to figure out the frontend...