Add .editorconfig

There were a couple of recent threads that wanted to add an 
.editorconfig file but never actually ended up doing so.[0][1]  Here is 
a patch.  It is meant to match more or less what's in .dir-locals.el.

I have only tested this with the GitHub view, not with an actual editor.

[0]: 
https://www.postgresql.org/message-id/flat/20180605172252.GA2433%40fetter.org
[1]: 
https://www.postgresql.org/message-id/flat/20180529153617.c72llpzbvt46chr6%40alap3.anarazel.de

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
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