Restrict search to certain namespace

[mfnalex]

The old mappings viewer allowed to search in specific namespaces (Spigot, Mojang, Obfuscated, ...) while the new viewer doesn't allow this. It'd be nice if the filter could get added to the new version, too.

[Misat11]

Currently you can search in specific namespace by writing ns:<namespace> (eg. ns:spigot) before the searched term.

[zlataovce]

I plan to add a visual guide to the site that explains that and/or add it to the planned documentation (#9), but it is currently explained at the index page of the site.

[zlataovce]

After more consideration, a namespace-choice component is more favourable for site users (multi-select, you'll still be able to search within all namespaces). This will be added to the site before the initial release and NMSMapper deprecation.