Log21

Added `Print` logging level.

Minor improvements.

Added a new method to `log21.Logger` class: `log21.Logger.clear_line`. This method clears the current line in the
console and moves the cursor to the beginning of the line.

Fixed a bug that would cause an error creating a progress bar with no value set for width in systems without support for
os.get_terminal_size().

Added `additional_variables` argument to `log21.ProgressBar` class. You can use it in order to add additional variables
to the progress bar:

python3
import log21, time

progress_bar = log21.ProgressBar(format_='Iteration: {i} {prefix}{bar}{suffix} {percentage}%', style='{',
additional_variables={"i": 0})

for i in range(100):
progress_bar(i + 1, 100, i=i)
time.sleep(0.1)
Iteration: 99 |██████████████████████████████████████████████████████████████████████████████| 100%

Added `formatter` argument to `StreamHandler` and `FileHandler`. You can use it to set the formatter of the handler when
you create it. Added `handlers` argument to `Logger`. You can use it to add handlers to the logger when you create it.