Feature/dynamic message

[joe-antognini]

This PR addresses Issue #53 by creating a DynamicMessage widget so that a user can supply some number (or numbers) on each update. This would be useful for machine learning applications where the user may want to keep track of some changing value like the accuracy or loss as a model is being trained. The widget could be used as follows:

import random
widgets = [Percentage(), Bar(), DynamicMessage('loss'), DynamicMessage('accuracy')]
with ProgressBar(widgets=widgets, max_value=2000) as pbar:
    max_so_far = 0
    min_so_far = 1
    for i in range(2000):
        val = random.random()
        if val > max_so_far:
            max_so_far = val
        if val < min_so_far:
            min_so_far = val
        time.sleep(.01)
        bar.update(i, accuracy=max_so_far, loss=min_so_far)

This PR is similar in purpose to PR #59 but it modifies update directly rather than adding an additional update_mapping function.

[wolph]

Thanks for the fix Joe :)

It's merged now and as soon as I have everything tested I will create a new release

[Kyalma]

Why does it support only interger (or float) value ? Couldn't we display a string ? I could change the code myself but I don't know if there is a length limitation

[wolph]

@Kyalma you can try the FormatLabel or FormatCustomText