Possiblity to add custom fields

[Paddy4711]

Dear Peppermint team,
first things first, thanks for your effort in creating an open source ticket system.

One thing that would really bring things forward, would be the possibility to add custom field to tickets. I.e. field like number/text/alphanumeric. Such fields would be handy if for instance there is a reference number by the client which could be recorded in peppermint.

Thanks in advance!