This is a rebase and slight refactor of #549 (without behaviour change) + documentation.

Original description from @tarasov65536:

This PR gives ability to specify set of filters to containers query when new generated file arrived.

It is kinda similar to #311 but with better implementation from architectural point of view.

It introduces new command line argument -notify-filter that adds filter in list of filters to query containers that should be notified when generated file changed. Argument format as in docker api

For example:

docker-gen -notify-filter name=foo -notify-filter label=bar=baz ...

adds two filters to containers query.

Signal code still defined by argument -notify-signal.

As pointed out in the original PR, docker-gen is not structured to make this realistically unit testable, but it work as advertised and does not seem to introduce regressions.

Resolves #77