main: Add .editorconfig.

This adds a .editorconfig file that specifies the canonical indentation
style and charset for various file types in the repository.

The vast majority of the files in the repository are Go code which is
required to use gofmt and thus almost all of the formatting is handled
by that.  However, additionally specifying a canonical indentation style
helps keep wrapping consistent among developers.

I am fully aware that some developers feel strongly about specific
amounts of indentation and canonical values are not going to please
everyone.  It is possible to override these settings for developers that
feel super strongly about it so long as the final code they commit is
consistent with them.

In any case, it is important to have a canonical standard to help
prevent developers, often inadvertently, rewrapping each others code due
to differing values.  It's the same reason that gofmt, and variants in
other modern languages that have followed suit, are so great.

.editorconfig

@@ -0,0 +1,17 @@
+# top-most EditorConfig file
+root = true
+
+[*.{go,gv,s}]
+indent_style = tab
+indent_size = 4
+charset = utf-8
+
+[*.py]
+indent_style = space
+indent_size = 4
+charset = utf-8
+
+[*.{js,json,sh,xml,yml}]
+indent_style = space
+indent_size = 2
+charset = utf-8