Generalize code and documentation to include Firefox

An [implementation for custom formatters in Firefox](https://bugzilla.mozilla.org/show_bug.cgi?id=1734614) is on its way. So the code and documentaton of the CLJS DevTools should account for that.

Currently, there's an [explicit check for Chrome 47 in the formatters code](https://github.com/binaryage/cljs-devtools/blob/4b4e256019f58ca7b3227a68ab2606664c95a906/src/lib/devtools/formatters.cljs#L17).

One possibility would be to simply remove the check and the logic around it as browsers not supporting custom formatters simply display the logged objects with their normal rendering.
The downside of that approach is, of course, that there won't be a hint anymore when the custom formatters couldn't be applied.

And the documentation is also still very much focused on Chrome as browser. Once the implementation in Firefox stands, all mentions in the documentation should reflect that. This means, adding Firefox where necessary or removing the wording related to Chrome where possible.

Sebastian