Fix line props

[d-fischer]

This PR is a two-for-one: it fixes two issues that keep me (and apparently others, as I found the issues to already exist) from working with lineProps in this library productively.

• It now passes lineNumber properly even if showLineNumbers is false
• It now merges the className property with the classes added internally instead of overwriting them

Fixes #288
Fixes #391

[d-fischer]

I updated the wrap-lines snapshot file. Now it looks like this fails the exact same tests as master, and I won't update these as the "new" results clearly seem broken (and, as I said, not by me).