Integrate grunt-newer as an option or a plugin

[searls]

Shopify has had great luck using grunt-newer in conjunction with their directory-based watching scheme. We should investigate whether this should be considered as an outright improvement over the current lineman-core behavior or whether we should build support as a plugin https://github.com/tschaub/grunt-newer

[davemo]

Tagging as a plugin enhancement; I agree this should be created as a plugin.