DOC: Improvements on logging

[willianpaixao]

Is this feature desired, fellow maintainers?

When cleaning old bundles, I had no idea which ones were being deleted and whether it was successful or not.
A few logging lines would be appreciated.

[freddiev4]

Hey @willianpaixao thanks for the PR!

Could you just post an example output for when you clean the bundles up?

[willianpaixao]

Here you go:

$ zipline bundles
csvdir <no ingestions>
quandl 2019-11-10 00:45:58.577633
quandl 2019-11-10 00:34:29.738155
quandl 2019-11-10 00:26:38.237694

$ zipline clean --keep-last 1
[2019-11-10 00:53:44.868754] INFO: zipline.data.bundles.core: Cleaning 2019-11-10T00;26;38.237694.
[2019-11-10 00:53:44.909732] INFO: zipline.data.bundles.core: Cleaning 2019-11-10T00;34;29.738155.

I've just rebased my branch to upstream/master to check if there won't be any problems merging an six months old branch. All tests passed.

I'm also removing the WIP tag since I don't see any more logs to be added in this specific command (zipline clean). If you find another place to add more log messages, I would be happy to implement.

[willianpaixao]

Hey @freddiev4 @richafrank can we get this PR merged please?

[ssanderson]

Hey @willianpaixao. Sorry for the long delay on this PR. I made a few small suggestions on your changes. In general, it's preferred to let the logging library to string formatting rather than doing it yourself. The main reason for this is that it avoids doing extra work for log lines that will be filtered out. In this case it's not a big deal performance-wise, but as a point of general style we try to avoid eagerly formatting log lines.

[willianpaixao]

Suggestions applied, @ssanderson.

[coveralls]

Coverage decreased (-0.5%) to 87.457% when pulling 769bc8e on willianpaixao:willianpaixao_logging-improvements into a1eb9b8 on quantopian:master.

[ssanderson]

thanks @willianpaixao