Allow configuration for includes - #428

I'm using ec-code-coverage in a monorepo with v1 addons and I see "unwanted guests" in my coverage, which are local addon dependencies. I figured it is far more straight-forward to constrain the include rather than doing exclude gymnastics.

On top, given how v1 addons are built, putting paths into excludes isn't going to be the path that is checked against. Logging here: https://github.com/istanbuljs/babel-plugin-istanbul/blob/37bb294cf433ca327572a2c3e1e4e6c32484c16e/src/index.js#L109 reveals this:

path/to/package/<pkg-name>/...
path/to/package/<dep-name>/...
path/to/package/<@scope/dep-name>/...

To ease gymnastics, this PR allows for explicit includes.

Let me know, what else is needed.