Highlighter plugin

Highlighter moved into a plugin.
Highlighting API changed. There are examples in plugin's readme. Also `demo/load_WP1_with_highlights.html` was updated.

#239 is solved by replacing new line chars (\n) with a space. It seems to be much easier to read and write `Label name` instead of `Label\nname`.

After merging this pull request most probably we'll have to bump Pathvisiojs to version 2.1.0 as it introduces new functionality. However it seems to be compatible with 2.0.x as in 2.0.x highlight method was not exposed through API.