cmdline entry format improvements

Created by: ederag

There have been requests for a more permissive parser (#280 (closed), #423 (closed)) and different suggestions such as activity name[@category[, description] [#tag1] ... [#tagn]] in https://github.com/projecthamster/hamster/issues/438#issuecomment-529860808, or name[@Category] [#tag1] [#tag2] ... [description] in https://github.com/projecthamster/hamster/issues/438#issuecomment-546314277.

My current opinion is that there is no way to have a smart parser using heuristics suitable for almost any input. This is even more difficult as this parser should keep the currently satisfied users experience intact.

Both the above suggestions do not keep compatibility. The first one because it would not be possible to enter activity, description without category. The second one because description would now be at the end. Another issue with the second one is that it would prevent entering a description starting with #.

So the plan could be to make all separators configurable, such as ,, before description. Regular expressions could be allowed. The defaults would not change, so most people would not even notice.