Uses `tune.run` to execute experiments as preferred API. @noahgolmant This does not break backwards compat, but will slowly internalize `Experiment`. In a separate PR, Tune schedulers should only support 1 running experiment at a time.
Adds support for custom loggers, custom trial strings, and custom sync commands. Closes #3034, #2985, and #3390.
Adds an example page and link in codebase. Closes #2728.
