Include .editorconfig

http://editorconfig.org/
Refined .editorconfig
Use* global rules with exceptions as needed
'root' repo end 'insert_final_newline = true'
Merge branch 'ntwb-editorconfig' into 3.0.0-wip
explicit callout in readme for building static assets; fixes #7632
shoutout to editorconfig in readme
Merge branch '3.0.0-wip' of github.com:twitter/bootstrap into 3.0.0-wip