matplotlib as extra requirement about nelson_siegel_svensson

[luphord]

I think the docs should make it clear that there are two different "modes" of installation: If you want to use nelson-siegel-svensson as a library, you can go with the ordinary pip install. If you want to use the command line interface, you need the "extras" version.
I am fine with this, but please update the docs (in particular installation.rst and README.rst) accordingly.

[luphord]

matplotlib is used in the command line interface to allow plotting via the nelson_siegel_svensson plot subcommand. That's why it is included as a dependency in setup.py.

I don't consider this feature "essential", so I am open to moving matplotlib to an extra dependency (probably click should then be treated the same way). @guym0r Would you care to submit a PR?

[guym0r]

Yes I would love to :)
But the installation command would change from

pip install nelson-siegel-svensson

to

pip install nelson-siegel-svensson[cli]

in order to run the CLI feature. (the original command will work but witout the CLI feature).
Are you sure you fine with this?