class documentation
        
        class ProgressBar:
A 2-line progress bar.
The progress bar looks roughly like this in the console:
                            Header
    20% [===========----------------------------------]
The progress bar is colored, if the terminal supports color output; and adjusts to the width of the terminal.
| Method | __init__ | Undocumented | 
| Method | clear | Clears the progress bar (i.e. removes it from the screen) | 
| Method | update | Updates the progress bar. | 
| Method | update | Updates the message of the progress bar. | 
| Constant | BAR | Undocumented | 
| Constant | HEADER | Undocumented | 
| Instance Variable | cleared | Undocumented | 
| Instance Variable | header | Undocumented | 
| Instance Variable | last | Undocumented | 
| Instance Variable | last | Undocumented | 
| Instance Variable | progress | Undocumented | 
| Instance Variable | term | Undocumented | 
| Instance Variable | width | Undocumented | 
Updates the progress bar.
| Parameters | |
| percent | the percentage to be shown. If None, the previous value will be used. | 
| message | the message to be shown above the progress bar. If None, the previous message will be used. |