feat: allow multiple -notify-sighup / -notify-container

[buchdag]

The -notify-sighup and -notify-container option were obviously planned to allow them to be passed multiple times, since they're internally stored as a map of container ID (string) to signal (int). This PR make it possible to use both of those options multiple times, like this:

docker-gen \
    -notify-sighup container1 \
    -notify-sighup container2 \
    -notify-container container3 \
    -notify-container container4 \
    -notify-signal 2 \
    ./some/template.tmpl ./result.ext

This would send HUP (1) to container1 and container2 and INT (2) to container3 and container4.

For notifying large amount of containers, the recently added -notify-filter is preferable.

There was also a TODO for preventing duplicate paths being passed to -config that this PR fixes.