Provide a more convenient way to look up users

The UUID is not a very easily accessible piece of data. It would be much easier to add default reviewers if we could use a more easily shared / maintained piece of information about a user like their username / email address.

```
data "bitbucket_user" "example" {
  nickname = "the-dude"
}
```


```
data "bitbucket_user" "example" {
  email = "the-dude@acme.com"
}
```