new: custom bundle build dir basenames via parseBundle() option parameter

Allow bundle build directory naming to be specified by a function
passed as a parameter to parseBundle via option object key named
"buildDirectoryNamer".

i.e.

    var loc = new Locator({buildDirectory: 'build'}),
        opts = {buildDirectoryNamer: function(bundle) {
                return bundle.name + '_' + bundle.version;
            }};

    loc.parseBundle(__dirname, opts).then(/* ... */);