Include .editorconfig

http://editorconfig.org/
Refined .editorconfig
Fixes #7577: Attempt to fix, at least, issues with Windows Phone 8 and responsive not kicking in correctly
typo
Fixes #7648: Thumbnail caption should use @text-color variable. Also adds other thumbnail vars
Fixes #7617: lowercase and dash gray color vars
Use* global rules with exceptions as needed
'root' repo end 'insert_final_newline = true'
Fixes #7661
Merge pull request #7662 from cvrebert/less-1.4-fixes

Fixes #7661
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
'root' folder .editorconfig code guide updates
'less' folder .editorconfig code guide updates
'js' folder .editorconfig code guide updates
'docs' folder .editorconfig code guide updates
Merge pull request #7664 from ntwb/editorconfig-3.0.0-wip

Editor Config (.editorconfig) Code Guide Update