New add-source command

This command adds source paths to the project under a given group name. Here's some example usage:

```
xcproj add-source Foo/Bar.* Foo
```

Will add all files matching that glob to the Foo group.

```
xcproj add-source Foo.* Bar/Baz
```

Will add all the matching files to the group Baz nested under Bar.

```
xcproj add-source Foo.* Bar.* root
```

Will add all matching Foo and Bar files to the root group of the project.

Please let me know if you see anything that can be improved here. I also tried to match the coding style so let me know if I need to change anything there. Thanks!