[feature] depsyncer add sync all dep

[JieAnthony]

@jessepeterson
New option --all is added to indicate operating all.
All I can think of is reading the database every time.
Although this may bring additional overhead

[jessepeterson]

Yeah -- You could definitely script this. If you're using MySQL just have it read-out the DEP names with mysql and restart itself every hour or something. Could also do that for the file backend. But I've noted this enhancement and we can consider the API support to query what we need and possibly implement this. Don't hold your breath, though. :)

This issue original mentioned in #42.