fixed panic when using mutators on floats

README.md

@@ -204,6 +204,26 @@ day: 2 | month: 1 | year: 2006
 day: 2 | month: 2 | year: 1962
 ```
 
+### Adding 2 values together
+
+Input:
+```
+5 7
+3 2
+10 152
+-15 3.7
+```
+
+Command: `format -i '(-?\d+) (-?\d+(?:.\d+)?)' -o '{1} + {2} = {1:%.2f:+(2)}'`
+
+Output:
+```
+5 + 7 = 12.00
+3 + 2 = 5.00
+10 + 152 = 162.00
+-15 + 3.7 = -11.30
+```
+
 ### Bulk renaming files
 
 Rename a bunch of files using format at once