Extend / replace "show language" to be more generic

[lukasoppermann]

As discussed in #910 it would be helpful if the show language plugin would be replaced by a more generic version that can add different custom labels to a prism instance.

Some features could be:

• add multiple custom labels
• auto-add language (if desired)
• add a div to collect all labels (could be used as a bar on top or bottom, but could also be ignored, so that individual items can be styled, whatever a user wants)
• if something like the config from Option to default to line-number for all code blocks #912 should be implemented, this plugin could use a config option to receive all items it should grab

There could be some overlap with the toolbar plugin from #891
The question is, if it would make sense to combine it with the toolbar or if they are to different in the idea. I think we should still try to keep it as small and simple as possible.

[mAAdhaTTah]

I think the main difference between the show-language plugin & the toolbar plugin is the styles, honestly. I do know, for now, they don't play well together, so perhaps one should supersede the other.

[mAAdhaTTah]

These have been combined now, so this is resolved.