Feature Request: Add flexibility to add a title string prefix for the progressbar

[element-jatin]

xlua.progressbar() currently lacks ability to add a title string as a prefix such as 'training', 'validation' or 'testing'.

Training : [=================== 5306/5306 =================>] Tot: 8m57s | Step: 107ms
Validation : [==================== 100/100 ==================>] Tot: 13s850ms | Step: 164ms

It is a simple fix to concatenate with title and adjust the progressbar width accordingly.