Add format_str parameter for progress bar and mill

[RazerM]

I wanted to add a way to customise the progress bar output, so implemented a parameter to pass a format string.

The details are in the commit message, but is shown here with formatting:

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progress.Bar, progress.bar, and progress.mill have a new format_str parameter.

Named fields are used to facilitate the change. Using the new syntax, the original formats could be represented as below:

'{label}[{bar}] {completed}/{total} - {eta}'
'{label} {mill} {completed}/{total}'

str.format() fails gracefully when named fields aren't present in the string, allowing a given part of the string to be omitted. On the other hand, incorrect field names cause a KeyError, preventing user error.

Note that when format_str=None, the old templates are used so that any code that modifies progress.BAR_TEMPLATE, for example, is not affected by the change.

Also, the format_str parameter does not require \r at the end, it is appended automatically.

[0xallie]

Yes please. Is there anything preventing this from being merged (other than the conflict in AUTHORS that is trivial to resolve)?

[0xallie]

Redundant merge commit much. :P You should have used git pull --rebase.

[RazerM]

I clicked the resolve conflict button in the UI since you mentioned it. I'll bother rebasing if it a) looks like it'll be merged and b) won't be squashed…