installer: use tabs to allow future heredocs

This will allow us to use tab stripping heredocs with `<<-'.
See http://www.tldp.org/LDP/abs/html/here-docs.html#EX71A

Add editorconfig file to enforce this style.
See http://editorconfig.org for more information.

.editorconfig

@@ -0,0 +1,10 @@
+root = true
+
+[*]
+end_of_line = lf
+insert_final_newline = true
+charset = utf-8
+
+[*.sh]
+indent_size = 4
+indent_style = tab