Support multiple key constraints on a single attribute

[lnschroeder]

When a column is a primary key and foreign key at the same time, only the PK constraint will be shown, even though mermaid supports PK,FK inside the constraint column:

erDiagram
    CAR ||--o{ NAMED-DRIVER : allows
    CAR {
        string registrationNumber PK
        string make
        string model
    }
    PERSON ||--o{ NAMED-DRIVER : is
    PERSON {
        string driversLicense PK "The license #"
        string name
    }
    NAMED-DRIVER {
        string carRegistrationNumber PK, FK
        string driverLicence PK, FK
    }

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I will create a PR, as this is a necessary feature to start with the PR for #43.

[KarnerTh]

Now available in version 0.10.0