Add --format option to control output

[datapythonista]

Would be useful to have an isort option to specify the format of the output.

For example, with flake8 the next can be used:

flake8 --format="##vso[task.logissue type=error;sourcepath=%(path)s;linenumber=%(row)s;columnnumber=%(col)s;code=%(code)s;]%(text)s"

And the output will have the specified format. This would be useful for example for CI systems, where log lines in specific formats are known to be errors or warnings (the previous example would make Azure pipelines consider linting issues in flake8 output as errors).