Add support for setting/changing total after creation

In some cases, like HTTP downloads (see [`progress_function`](https://docs.rs/curl/0.4.4/curl/easy/struct.Easy.html#method.progress_function) in libcurl for example), you may not know the size of the download until after you have been in touch with the remote server. But you may still want to create the progress bar to show that things are happening. Once the connection is established, and the `Content-Length` header has been received, the total for the progress bar can be set, and progress can be logged.

It'd be great if `pb` offered a way to create a new, "indeterminate" progress bar, and then allow for the total to be set *afterwards*.