Some plugins define neo-tree sources. For theese plugins having an
option 'neoTreeIntegration = true' would be useful. The problem is that
if we set the plugins.neo-tree.sources to ["<source>"] then we ditch the
default values of ["filesystem" ...]. The idea here is to define an
internal extraSources option that takes care of the following:
- If sources is null set it to the default sources
- Concatenate sources and extraSources