add .editorconfig

see: http://editorconfig.org/
(cherry picked from commit b9b6f42dd13fe326b0271bdff945833c6bfd9080)
[bug] fix lookbehind assertion fails with /m mode enabled (Issue #15)

Optimization for .* should be disabled if lookbehind assertion is used.

see: http://bugs.ruby-lang.org/issues/8023
(cherry picked from commit 83a0bd029eb5a5a4986156490c578941d7ec7ca1)