Consider unique constraint on user_id and name

[MikeGlennFWC]

I feel there is a benefit to adding a unique constraint across the columns user_id and name.

Currently users can create multiple tokens with the same name, meaning differentiating between them at a later date would be impossible as the key is only to be shown once.

[driesvints]

Duplicate of #25

[MikeGlennFWC]

Can this be reviewed again, #25 has been closed based on the first issue.

[driesvints]

@MikeGlennFWC we won't be enforcing this through the library. You can implement this yourself in your app if you want.