#19 specify time period for export

[Yurkennis]

Would like to specify period for export -- for me, 'Since
[Date]' would suffice.

This would make MsgExport more useful for performing
global search in history -- typically you need to search
only in recent messages.

[Kennet]

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user_id=496309

do you delete all the files every time you are done searching ?

[Yurkennis]

Logged In: YES
user_id=144045

I haven't used MsgExport for GlobalSearch frequently yet.

And I assumed that MsgExport doesn't update the exported
history real-time. Now I see that it does update, which makes
this feature request far less critical.