Issue title: Pre- and -after meal
 Issue body: My glucometer requires me to select if the value refers to a pre- or after- meal tracking. It would be great to have this feature in glucose-tracker as well.
 Comments: 
 Comment 0: Hi Angelo,

One way to handle this is to use tags.  You can then filter your data by your chosen tags in https://www.glucosetracker.net/glucoses/filter/
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