Add isort.force-sort-within-sections setting (#1635)
This commit is a first attempt at addressing issue #1003.
The default `isort` behavior is `force-sort-within-sections = false`,
which places `from X import Y` statements after `import X` statements.
When `force-sort-within-sections = true` all imports are sorted by
module name.
When module names are equivalent, the `import` statement comes before
the `from` statement.
