mercurial: add editorconfig

Editorconfig (http://editorconfig.org/) is a file format helping define coding
styles like spaces, tabs etc. It supports a wide range of editors. Some well-
known projects like ruby and zsh are using it already.

This patch adds a simple .editorconfig, making it clear we use 8-char tabs in
C code, 4-char spaces in Python code, and we don't keep trailing spaces.

.editorconfig

@@ -0,0 +1,13 b''
+# See http://EditorConfig.org for the specification
+
+root = true
+
+[*.py]
+indent_size = 4
+indent_style = space
+trim_trailing_whitespace = true
+
+[*.{c,h}]
+indent_size = 8
+indent_style = tab
+trim_trailing_whitespace = true