Fix double-formatting client-side

[peaBerberian]

This is an alternative attempt (after
#22) at fixing the "double formatting" situation brought by
canalplus/rx-player#1469 that may be made much more frequent by canalplus/rx-player#1625.

This solution fixes it client-side instead, which could be seen as arguably less ugly.

The idea is to rely on the fact that the RxPlayer does full formatting by calling log functions with at least three arguments:

1. The timestamp in a string format with always numbers after a comma
2. A "namespace" (e.g. "[warn]") 3-n. The log message's arguments

By considering this, we can very easily detect client-side a probable case of full formatting.