report coverage delta between two coverage files

This allows to compare 2 coverage files and report the coverage difference.

Same coverage:
```
Name         Stmts    Miss    Cover    Missing
-----------  -------  ------  -------  ---------
dummy/dummy  -        -       -
TOTAL        -        -       -
```

Better coverage:
```
Name         Stmts      Miss  Cover    Missing
-----------  -------  ------  -------  ---------
dummy/dummy  -            -2  +50.00%  -2, -5
TOTAL        -            -2  +50.00%
```

Worse coverage:
```
Name         Stmts      Miss  Cover    Missing
-----------  -------  ------  -------  ---------
dummy/dummy  -            +1  -25.00%  +2
TOTAL        -            +1  -25.00%
```