Allow capturing logs as string

Describe the new feature
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Similar to #188, I want to be able to capture the PuLP output and put it all in a single log line, rather than having it go to stdout directly. Instead of creating a temporary file and pointing PuLP to the log file, I would like to be able to get the log output as a string, similar to how you can do it with [subprocess.check_output()](https://docs.python.org/3/library/subprocess.html#subprocess.check_output). Alternatively, we could provide a file object (like StringIO) if that's easier.

Additional info
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Please answer these questions before submitting your feature request.

#### Is your feature request related to an issue? Please include the issue number.
#188

#### Does this feature exist in another product or project? Please provide a link.
Python has a native way of getting output from subprocess command calls: https://docs.python.org/3/library/subprocess.html#subprocess.check_output

Start by reading issue #188 and the solver output handling that currently writes to stdout or a temporary log file. Compare the requested behavior with Python's subprocess.check_output() and a StringIO-style file object. Done means PuLP exposes a way to retrieve the complete solver output as a string or file-like object instead of sending it directly to stdout.

Written by the indexing model from the issue text.