Fix BEM convention for SelectorFormat

Rename the `BEM` convention for the `SelectorFormat` `convention` option
to `hyphenated_BEM`, and introduce `BEM` format as specified in
https://bem.info/method/definitions/

Change-Id: Id7764250bfc4c10d1d46374bd6d4e217f170c8f5
Reviewed-on: http://gerrit.causes.com/44135
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>