support globbing for flat-files arguments

[he2ss]

It will be useful if the --flat-file argument can support globing, nxapi will be able to process many files at the same time.

example : python nxtool.py --flat-file /mydirectory/error.log.*

[jvoisin]

Blocked by #16

[jvoisin]

On the other hand, what about using something like python nxtool.py --stdin <<< $(cat /mydirectory/error.log.*) instead?

I don't think that we want that kind of logic inside nxtool-ng.