Add a progress bar

[joserobjr]

Is your feature request related to a problem? Please describe.
Currently it's impossible to know the progress that it has made and how long it is going to take. Neither by command line nor by API.

Describe the solution you'd like
Add some properties to WorldConverter that will keep track of the progress and how long it is going to take, like the number of regions, chunks and an estimated time remaining. The properties should be accessed asynchronously from the caller because the convert method is a blocking operation. The WorldConverterCLI would consume that information and show them in console or in a small java swing GUI.