Introduce styleguide to avoid unnecessary merge conflicts

[mightymichael44]

Hi @lava, thanks for the great project.

I just discovered that I would like to have both the master of this repo as well as the new features from #201
Since it has +/-1.5k lines it's clearly not mergeable, but while reading the changes I noticed a lot of the differences arise from different formatting...

What about introducing a stylefile, e.g. .clang-format file in whatever style you prefer and applying it here to the master.

Applying it on all incoming PR's would remove the clutter of different formatting.

P.S.: If you decide on a preferred codestyle I could open a PR, adding the .clang-format file and the formatted matplotlibcpp.h