WP-CLI: Be more explicit with output

[GaryJones]

Currently a command like wp msm-sitemap generate-sitemap-for-year-month --year=0217 --month=01

will generate a list like:

Generating sitemap for 217-1
Generating sitemap for 217-1-1
Generating sitemap for 217-1-2
Generating sitemap for 217-1-3
...

It would be more useful if it did something like:

Generating sitemap for 217-1
Generating sitemap for 217-1-1 - empty (not generated)
Generating sitemap for 217-1-2 - generated with 2 posts
Generating sitemap for 217-1-3 - empty (old sitemap removed)
...

...or similar to actually indicate whether a sitemap was generated or deleted. This could optionally be behind a -v verbosity flag.