--first-commit-add does not have any effect

Regardless if I put 0 or 1000 or nothing for first-commit-add it does not make any difference for the number of hours calculated.

```
$ git-hours
{
"X": {
"name": "X",
"hours": 233,
"commits": 1218
},
"total": {
"hours": 233,
"commits": 1218
}
}
$ git-hours --first-commit-add=0
{
"X": {
"name": "X",
"hours": 233,
"commits": 1218
},
"total": {
"hours": 233,
"commits": 1218
}
}
$ git-hours --first-commit-add=1000
{
"X": {
"name": "X",
"hours": 233,
"commits": 1218
},
"total": {
"hours": 233,
"commits": 1218
}
}
$ git-hours --version
1.5.0
```

It seems --max-commit-diff might have the same problem. Just installed git-hours with npm.

Start with the git-hours CLI handling for --first-commit-add and --max-commit-diff, then reproduce the reported commands using version 1.5.0. Trace where these options enter the hours calculation; done means changing either value changes the calculated hours when appropriate.

Written by the indexing model from the issue text.