Allow TPOT to run for the shortest (or longest) between generations and max_time_mins

When I set-up an experiment, I often find myself wanting to execute a set number of generations, but cut off if it exceeds X minutes.
For example, I want to run TPOT for 100 generations, but only use at most 60 minutes of time.
I can also imagine someone would want to do the exact opposite; run TPOT for at least 100 generations, but also run for at least X minutes of time.

As is, you can only pick between a set maximum of generations or a certain time.
When both are provided, the set amount of generations actually does nothing, and TPOT is run for `max_time_mins` (this is explicit in the code).

Besides the lack of above options, it also bothers me that `max_time_mins` does not so much specify a maximum time, but a set time for which the experiment will run (and, practically speaking, never shorter).

The changes are very minor in code (I did this for my own versions), but I wanted to discuss if this is a good addition, and how this should change the constructor parameters.