Add support for OpenSearch description format

[user451421541757324]

The OpenSearch description format can be used to describe the web interface of a search engine. This allows a website to describe a search engine for itself, so that a browser or other client application can use that search engine.

A quick example:

<OpenSearchDescription xmlns="http://a9.com/-/spec/opensearch/1.1/">
    <ShortName>libremdb</ShortName>
    <Description>libremdb, a free & open source IMDb front-end</Description>
    <InputEncoding>UTF-8</InputEncoding>
    <Url type="text/html" template="https://libremdb.iket.me/find?q={searchTerms}" />
</OpenSearchDescription>

[zyachel]

that's cool! i've added hardcoded only my instance for now. but will make it instance-specific later on