Adds scanning for sources never executed, fixes branch destination bugs

This release:

• adds --version to the command flags, to display SlipCover's version and exit;
• adds scanning for files never executed in the directories passed with --source, so that these, too, are included in the output;
• adds a summary line to the text version of the coverage results;
• fixes bugs computing the destination of certain branches when they occur within a try (including except, except*, try-else and finally). This affects only the branch destination shown, not their existence, counting, or statistics.