Add a `_trClass` and / or `_trAttr` property to <b-table> items

[kbirk]

Currently there are the tdClass and tdAttr properties for the table fields. It would be really nice to have something similar for the table items. Such as _trClass or _trAttr per item which would be injected in each row in the table.

My use case requires injecting a row-unique value into each row's DOM element to interface with another 3rd party library. This row-unique value is not displayed in the table so I can't acheive this in a roundabout way using the existing tdClass / tdAttr callback functions.

[tmorehouse]

Use the table primary-key option to enable a unique ID per row, which is based on the table's ID and the value of the field that is specified by the primary-key. The primary key field needs to exist in your data, but doesn't need to be displayed in the table itself.

[tmorehouse]

BootstrapVue v2.2.0 has been released.