* Make `user` default to `False`, as otherwise it conflicts with other custom install options like `nbextensions_dir` and `prefix`. This makes app default to system install.
* alter tests to account for new default install
* alter tests to check *all* potential installation directories, to ensure we don't place anything in a location we shouldn't touch
* test app `--prefix=` cli arg
* consolidate common operations into functions
* test imports in test_preprocessors
* skip rather than fail on tests which require pandoc if we can't find it
* move svg2pdf inkscape-finding into a function which doesn't raise exceptions, allowing us to skip test if inkscape can't be found, and skip accordingly
* use urllib instead of requests in svg2pdf preprocessor
* fix flake8 complaints
* rename `jupyter_contrib_nbextensions.extensions` -> `jupyter_contrib_nbextensions.nbconvert_support`, since they're now all nbconvert things
* rename `tests/test-postprocessors.py`, since it actually tests preprocessors!
* move highlighter config scripts into `config_scripts` package
* remove redundant scripts in `src/jupyter_contrib_nbextensions/nbextensions/highlighter`
Juergen Hasch