Add methods to manipulate Summary Word Count

[aerkiaga]

This adds methods word_count, set_word_count and clear_word_count to SummaryInfo, which can be used to set the Word Count field within the Summary. This is necessary for many use cases, since the default value is 0, while a typical installer containing compressed files should set it to 2.

[mdsteele]

Looks good, thanks! Could you please exercise these new methods in the set_properties() test at the bottom of the file? Also I think you'll need to run cargo fmt to make the linter happy.

[mdsteele]

Thanks!

[mdsteele]

I've just published msi v0.6.0 with this new feature.