Add mysqldump example command

[rfay]

The Problem/Issue/Bug:

#1652 requested additional flags for import-db and export-db, but import-db and export-db may be mostly eclipsed by new custom commands mysql and (in this PR) mysqldump.

How this PR Solves The Problem:

Adds ddev mysqldump example command (must be enabled)

Manual Testing Instructions:

cd .ddev/commands/db
ln -s mysqldump.example mysqldump
ddev mysqldump db | gzip >/tmp/db.sql.gz

Automated Testing Overview:

Related Issue Link(s):

Release/Deployment notes: