Add option for usernames or omitting user info in change files/changelogs

[ecraig12345]

Currently, change files and changelogs contain the user email. This is an artifact of the change file concept being copied from Rush, which was primarily used with internal repos where it made sense to list everyone's emails in the changelog. This makes a lot less sense for public repos.

The options needed are:

• Usernames (e.g. @ecraig12345) instead of emails. The challenge is there's no standard git config for the github/etc username (not to be confused with user.name). git config allows arbitrary keys, so we could probably add our own like beachball.username. This would probably also require a separate step in the change prompt to set the username (local or global) if it isn't already done.
• Entirely disable collection/writing of user info

This could probably be handled with a new config option like userType: 'email' | 'username' | 'none'.

(#375 points out a workaround for hiding emails from the changelog md, but this doesn't prevent them from being recorded in change files.)