Provide the ability to control the source of time

I had been looking at this library as a library I could use to control the "current time" by controlling a "managed clock": https://hexdocs.pm/time_machinex/readme.html#content

However, this library is not compatible with TimeX due to the overlap in domain.

* Be able to use the clock as usual when in `:dev` or `:prod`, for example, but when in `:test` the clock would behave differently (or have a flag to the `now()` function that uses the managed clock)
* Be able to freeze the clock and restart it
* Be able to set any arbitrary time in any TZ on the clock
* Be able to speed up or slow down time by a factor, such as 1 second per minute (slow by 60x) or 1 hour per second (speed up by 3600x)
* Be able to provide a function to the managed clock that transforms the real-time clock into another time (maybe it would be "good enough" to provide a way to pass a function into the current `now()` function and then use that function to transform the clock, then I can just put transformations into the code where I want it and we wouldn't need to have other methods to control a managed clock?)

Thanks so much for the nice library and for considering this proposal.