Add UI to filter features by type

[SebastianZ]

Let users choose between different types of features they are interested in. This seems to be already implemented via filtering by the categories you can specify for a feature, though that feature is not obvious. There should be some UI allowing to select one or multiple of the predefined categories.

Those may reflect the ones on MDN web docs, i.e:

• HTML
• CSS
• JavaScript
• Graphics
• HTTP
• APIs / DOM
• (Browser extensions)
• (MathML)
• Others

This could be implemented in different ways, e.g. as a dropdown list, a button bar, a link list or autocompletion feature while typing.

This is similar to the filter requested in #60 with the difference that there the features are grouped by (higher level) interest while here they are grouped by technology. The filter mechanism could cover both, though.

Sebastian