Implement account.username

[gr2m]

As described in https://github.com/hoodiehq/hoodie-client-account#accountusername

See how hoodie.id is implemented in hoodie-client for comparison

• [x] start a pull request
• [x] write tests for account.username
• [x] implement account.username
• [x] write tests for admin.username
• [x] implement admin.username
• [x] review & merge

[gr2m]

I'll take this one as I need it for a project by Sunday