Merge upstream - #1

* Add support for stem and xkcd
* Add README for xkcd

…f the Python version 3 and void if the Python version is 2.

At the moment, the errorbar() function takes an optional parameter s,
which is presumably supposed to correspond to the same parameter for
e.g. the plot function. The only problem is that a) this argument is
unused in the function and b) matplotlib's errorbar() doesn't take a
parameter like this anyway.

I've replaced the s argument with an optional keywords argument, as for
other functions, which allows for setting colour, marker style etc.

…ple of them to work. In the future matplotlibcpp should use map<string,any> instead of map<string,string> for kwargs. That should be a separate PR

The Python interpreter can be constructed by any thread while the destructor is always the main thread. This can lead in some errors with the Python side of things, where some objects want the thread which constructed them to also destroy them. By adding this 'kill' function, a developer can use your library (btw gj! 👍 ) with `std::thread` in order to create a plot async -aka without blocking the main thread- and then close the plot and 'kill' Python interpreter afterwards, without waiting the program to end or having destructor errors.

Doing this in order to showcase this more. Small change but not that polished. Let me know about your opinion first and we can fix it 😃.
Thank you for your time and this great library ❤️

…and make rcparams changeable.

See https://en.cppreference.com/w/cpp/string/basic_string/stof for details.

Memory is allocated with new and delete is never called. Use a std::array instead, so no memory will be allocated.

After the switch to cmake, we don't need the hand-written Makefile
nor the contrib/ version of the cmake file.

…nputs.