Adding a default config for npmlog.heading

Simply setting up a config that controls the log heading instead of it being hardcoded to `npm`.

If this request is approved, I'll submit another PR to `npm` to add the config here:
https://github.com/isaacs/npm/blob/master/lib/npm.js#L287

I have also added tests for this change.

The use case is that others have "wrapped" versions of `npm` and it would be nice from the output to tell which `npm` they are using.

Especially if they have aliases for different registries, they could add `--heading foo` to their alias and the log output will show which "alias" they used.