added new ipynb_cite option that emulates latex plain style citations

[simetenn]

The current latex option does not work very well. This pull request adds an additional option to --ipynb_cite=latex-plain that emulates the latex plain citation style. This solves the citation part of issue #101.

New Latex-plain style:

This is a citation [1].

References

1. A. (Andrea) Saltelli. Global Sensitivity Analysis : the Primer, John Wiley, 2008.

Plain style:

This is a citation [saltelli_global_2008].

References

1. A. (Andrea) Saltelli. Global Sensitivity Analysis : the Primer, John Wiley, 2008.

[KGHustad]

Looks good