Add support for allowlist filter

[willhardy]

Currently it is not possible to exclude unknown tables. This change allows tables to be explicitly listed so that only those tables will be dumped.

For example, the following will only export the customer table.

[filter]
* = ignore
customer = include

(The existing code is hardly touched, this is mostly just additions. However, one line is slightly refactored to early-exit, so the remaining code in the function ends up being de-indented.)