Support for BurntSushi/erd relationship syntax

[stijnherreman]

https://github.com/BurntSushi/erd uses ? instead of 0 for zero-or-one, but the current relationship regex does not recognise that syntax. Would you mind changing it from ".*[10\\*\\+]--[10\\*\\+]\\s*\\w+" to ".*[10\\*\\+\\?]--[10\\*\\+\\?]\\s*\\w+"?

Some context: https://github.com/kaishuu0123/erd-go is based on https://github.com/BurntSushi/erd and https://github.com/unok/erdm.

• erd-go uses 0for zero-or-one
• erd uses ? for zero-or-one
• erdm uses 0..1 for zero-or-one
• https://en.wikipedia.org/wiki/Cardinality_(data_modeling) lists 0..1 and ? for zero-or-one

[mikkel-ol]

I updated to version 0.4.0 with support for all cardinalities.

Thank you for your feedback.