add missing dep, improve center mixin

Author: Nicolas Cusan

.editorconfig

@@ -11,6 +11,3 @@ indent_style = space
 insert_final_newline = true
 trim_trailing_whitespace = true
 end_of_line = lf
-
-[*.md]
-trim_trailing_whitespace = false

package.json

@@ -29,6 +29,7 @@
     "autoprefixer": "^9.1.5",
     "babel-eslint": "^10.0.1",
     "babel-loader": "^8.0.4",
+    "beeper": "^1.1.1",
     "browser-sync": "^2.26.0",
     "cssnano": "^4.1.4",
     "del": "3.0.0",

src/sass/lib/mixins/_center.sass

@@ -3,7 +3,8 @@
 //============================================
 
 =center($width, $height: $width, $position: absolute)
-  +pos(50%, top left, $position)
+  @if $position
+    +pos(50%, top left, $position)
 
   +size($width, $height)
   margin-left: -$width / 2