Allow to easily skip some database tables

The new option allows to easily skip the content of a database table, for example
because the database table is SUPER HUGE and with temporary data.

The schema of these ignored tables will be saved. You're welcome.

Example usage:

skip_database_table_data "FOO_DATABASE" "FOO_TABLE1"
skip_database_table_data "FOO_DATABASE" "FOO_TABLE2"
skip_database_table_data "FOO_DATABASE" "FOO_TABLE3"

See also the example file.

The option is completely compatible with:

• backup_database "$db"
• backup_every_database
• backup_phabricator

For obvious reasons, it's important that you declare your skip rules BEFORE the above commands.

Closes T2400