Make it clear that only open issues are imported when importing all

The function `import_some_issues` can still import closed issues if
asked to.
Change to use actual config file

The `config` no longer allows any Python code in it! :D

Now it's a real, full-fledged config file.
Add ability to pass arguments

The following arguments are allowed (see descriptions of these arguments
by using the `--help` flag):

 * -h,          --help
 *              --config CONFIG
 * -u USERNAME, --username USERNAME
 * -p PASSWORD, --password PASSWORD
 * -s SOURCE,   --source SOURCE
 * -t TARGET,   --target TARGET

To make things easier, I renamed "destination" to "target" (this
away it matches other terminal commands a tad better).

Fixes https://github.com/IQAndreas/github-issues-import/issues/5