Add support for coverage files relative to shifter config

I've recently been trying to build coverage reports from shifted modules and hit this issue on the way.
At present, all coverage files are built with the name and filename set as:
```
'build/' + filename + '/' + filename + '.js'
```

When istanbul then comes along to produce the report, it tries to find the source files at this path; however, when building in a recursive or distributed situation and where build is not in your immediate path, this leads to unresolvable .js files.

This change sets a relativeTo option to shifter which is then used when instrumenting the -coverage.js files so that each file is built relative to the configuration.

In a nested structure such as:
```terminal
.
├── .shifter.json
└── path
    └── to
        └── yui
            ├── build
            │   └── somemodule
            │       ├── somemodule-coverage.js
            │       ├── somemodule-debug.js
            │       ├── somemodule-min.js
            │       └── somemodule.js
            └── src
                └── somemodule
                    ├── build.json
                    ├── js
                    │   └── somemodule.js
                    └── meta
                        └── somemodule.json
```
Instead of the path being set to:
```
build/somemodule/somemodule.js
```
It will be set to
```
path/to/yui/build/somemodule/somemodule.js
```

When it comes to creating the istanbul reports, this is now resolvable.