bridge: importing events occurred after a known date

[a-hilaly]

For now the importer can only iterate over all available issues/comments/events and import them if they don't exist locally .

We can add two flags to the bridge pull and push subcommands in order to allow only importing and exporting new events:

1 - --no-resume

for example, by default the importer will have to import only new events. passing this flag will force the bridge to import all available issues and events.

example: git bug bridge pull --no-resume

2 - --since

to import/export events after a known date
example: git bug bridge push --since=2019-06-08T21:49:06Z

[MichaelMure]

Possibly, we would also need a --since-like flag that understand the Lamport time.

Also, the importer/exporter should probably be smart enough to resume where it left off on it's own.