Add support for pt and pc (#87)

* Add support for pt and pc

* Misc fixes

inkstitch.py

@@ -102,9 +102,17 @@ def convert_length(length):
     if not units or units == "px":
         return value
 
+    if units == 'pt':
+       value /= 72
+       units = 'in'
+
+    if units == 'pc':
+       value /= 6
+       units = 'in'
+
     if units == 'cm':
        value *= 10
-       units == 'mm'
+       units = 'mm'
 
     if units == 'mm':
         value = value / 25.4