Add a general constructor with default values for all arguments

[Cyrilvallez]

Hi !

I like this package as I'm a huge fan of tqdm in Python. However, I find it very strange that you did not add a default constructor requiring an iterable, and providing default values for all arguments. Something like

ProgressBar(wrapped::Any;  arg1=foo, arg2=foo,...)

for all fields of ProgressBar. Indeed, the only outer constructor provided is not sufficient. For example, I wanted to set the description of a bar such as

ProgressBar(1:100, description="foo")

but the current state of the constructors will not allow me this simple syntax. In the current state, one would need to create a new ProgressBar (or redefine a constructor somewhere) by setting all fields just to put the description, which is absolutely not convenient.

If you want, I could quickly add the change in the source code and add a pull request.