introduce coding style standard for easier to read pull requests

[hennr]

As pull requests are harder to read on github's web ui if spaces are replaced with tabs or vice versa I suggest to introduce a standard for that.

editorconfig seems to be an option. We could add a file (.editorconfig) to the repo which is picked up by some IDEs (e.g. intelliJ) per default. Many other IDEs are supported via plugins.

I would start with this config and see how it goes if there are no vetoes:

root = true

[**]
indent_style = space
indent_size = 4
end_of_line = lf
charset = utf-8
trim_trailing_whitespace = true
insert_final_newline = true

[hennr]

I pushed the suggested config.

@mpatric Do you want to reformat or should I do it?

[mpatric]

@hennr you can do it

[hennr]

I just did the reformat and noticed that this is a big commit.

Tabs are used more often than spaces in the project so far, so I changed the .editorconfig to use tabs.
This leads to a smaller commit and thus preserves more of the old commit messages.