use bl@5's isBufferList to detect buffer list

`instanceof` doesn't work when using different instances of the
BufferList class, for example when different versions of buffer list are
used.

This commit upgrades to bl@5 which adds `isBufferList` which should work
across versions of `bl`.

I also moved `bl` to a peer dependency so it's easier for consumers to
match the bl version, and bumped the version to 6.0.0 because npm
doesn't install peer dependencies by default which could break consumers
without a bl dependency.

fixes #104