Merge [browsing] and [multiple-browsers] into [browser]

Question

I think these tags are too similar to be different tags. browsing and browser are basically the same, and if the purpose of multiple-browsers is to be more general (not browser-specific) then I think browser is general enough. If it's something else (maybe using multiple browsers simultaneously for something) then we can clean some of the questions tagged with it, but I think it's pretty useless.