Generate statistics reports from 'build' sub-command

[uribench]

Create an entry in a common log file with statics when running the 'handbook build' command.
The statistics entry may include:

• date/time stamp
• number of contents documents classified as config/metadata, Guide, and Topics.

This will help analyzing trends of contributions.

Implementation guidelines:

• This is not replacing the capabilities of the 'status' sub-command that supports various reports.
• The 'status' sub-command and this log generation of 'build' sub-command should share common code from lib.
• Add a '--no-log' option to optionally suppress the log.