Universal progress bar

There should be a progress bar for all long-running tasks

- [x] Implement generic progress bar class in C++, and supporting functionality;
- [x] Allow nested work objects
- [x] Replace progress bar in `fread()`
- [x] Add progress bar into `.to_csv()`
- [x] Replace progress bar in Aggregator
- [x] Add progress bar in Ftrl
- [ ] Add progress bar for generic `DT[i, j, ...]`
- [ ] Add progress bar for sorting
- [ ] Add progress bar for converting into pandas/numpy
- [ ] Add progress bar for converting to python
- [ ] Add progress bar for converting from pandas
- [ ] Check that progress bar works properly in Ipython
- [ ] Check that progress bar works properly in Jupyter
- [x] Add tests