Lots more implementation

* implement middle+/-
* cells unit
* samples for both
* moved shorthands to the units/ field

Note: cell_px is currently unconfigurable

lib/squib/args/unit_conversion.rb

@@ -1,9 +1,10 @@
 require_relative '../constants'
 
+
 module Squib
   module Args
     module UnitConversion
-      module_function def parse(arg, dpi=300)
+      module_function def parse(arg, dpi=300, cell_px=75)
         case arg.to_s.rstrip
         when /in$/ # ends with "in"
           arg.rstrip[0..-2].to_f * dpi
@@ -15,6 +16,8 @@ module Squib
           arg.rstrip[0..-2].to_f * dpi * INCHES_IN_CM / 10.0
         when /deg$/ # ends with "deg"
           arg.rstrip[0..-3].to_f * (Math::PI / 180.0)
+        when /c(ell)?[s]?$/ # ends with 'c', 'cell', or 'cells'
+          arg.rstrip[0..-2].to_f * cell_px
         else
           arg
         end

lib/squib/args/xywh_shorthands.rb

@@ -7,7 +7,11 @@ module Squib
       HEIGHT_MINUS_REGEX = /^height\s*\-\s*/
       WIDTH_DIV_REGEX = /^width\s*\/\s*/
       HEIGHT_DIV_REGEX = /^height\s*\/\s*/
-      
+      CELL_REGEX = /^c[ell]?[s]?\s*/
+      MIDDLE_PLUS_REGEX = /^middle\s*\+\s*/
+      MIDDLE_MINUS_REGEX = /^middle\s*\-\s*/
+
+
       # dimension is usually either deck_width or deck_height
       def apply_shorthands(arg, deck, axis: :x)
         dimension = (axis == :x) ? deck.width : deck.height
@@ -33,7 +37,15 @@ module Squib
         when HEIGHT_DIV_REGEX # e.g. height / 3
           n = (arg_s.sub HEIGHT_DIV_REGEX, '').to_f
           deck.height / n
-        else 
+        when MIDDLE_PLUS_REGEX # e.g. middle + 1.5in
+          n = arg_s.sub MIDDLE_PLUS_REGEX, ''
+          n = UnitConversion.parse(n)
+          dimension / 2.0 + n
+        when MIDDLE_MINUS_REGEX # e.g. middle - 1.5in
+          n = arg_s.sub MIDDLE_MINUS_REGEX, ''
+          n = UnitConversion.parse(n)
+          dimension / 2.0 - n
+        else
           arg
         end
       end

lib/squib/dsl/units.rb

@@ -28,5 +28,10 @@ module Squib
       n.to_f * (Math::PI / 180.0)
     end
 
+    # DSL method. See http://squib.readthedocs.io
+    def cells(n)
+      n.to_f * @cell_px
+    end
+
   end
 end