Add .editorconfig to aid collaboration

Fuzzyy has used 4 space indent for vimscript. There is no hard rule
here, but 2 space is probably more common (Vim's own runtime files are
mostly 2 space indent, Google VimScript style guide suggests 2 spaces),
so better to make this explicit for people who don't use anything to
perform automatic detection, e.g. vim-sleuth or vim-polyglot plugins.

Also, as the README is now being used as the source to generate the vim
docs, a max line length helps write documentation that looks similar
both in the source README and the generated vim docs.

Author: mmrwoods

.editorconfig

@@ -0,0 +1,14 @@
+[*]
+charset = utf-8
+end_of_line = lf
+indent_size = 2
+indent_style = space
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.vim]
+indent_size = 4
+
+[*.md]
+max_line_length = 80
+trim_trailing_whitespace = false