Consider a volo.includes or volo.files

[jrburke]

As opposed to excludes, the opposite. npm has a files which allows only keeping explicit files, in a nested directory search, which is a bit different than a pure opposite of volo.excludes, which only goes by top level entries. Marking for 0.3.2, but may get pushed for semver reasons, if 0.3.2 does not turn into an 0.4.0.