journal: ability to set flags for journal instance

[taranovegor]

I needed to merge journals, which involves using the --merge flag when calling journalctl. Since I couldn't find a way to do this in the current implementation, I propose the following solution.

The structure JournalReaderConfig is augmented with a field Flags that contains an array of constants. If the Flags array is not empty, the method NewJournalWithFlags is called with the bitwise OR of the values in the array.

sdjournal.NewJournalReader(sdjournal.JournalReaderConfig{
    Since: time.Duration(-1) * time.Minute,
    Flags: []int{sdjournal.SD_JOURNAL_SYSTEM, sdjournal.SD_JOURNAL_CURRENT_USER},
})