report coverage delta between two coverage files

This allows to compare 2 coverage file and report the difference:

```
Name                   Stmts    Miss  Cover    Missing
-------------------  -------  ------  -------  -------------
Main                      +0      +0  +0.00%
search.BinarySearch       +0      +0  +0.00%
search.LinearSearch       +0      -3  +15.24%  -19, -20, -24
TOTAL                     +0      -3  +5.00%
```

```
Name                   Stmts    Miss  Cover    Missing
-------------------  -------  ------  -------  -------------
Main                      +0      +0  +0.00%
search.BinarySearch       +0      +0  +0.00%
search.LinearSearch       +0      +3  -15.24%  +19, +20, +24
TOTAL                     +0      +3  -5.00%
```