Define common dependency tasks

[guybedford]

When a task has a long intricate list of dependencies, it can be useful to support a wrapper task which acts as a single node to capture those dependencies:

[[task]]
name = 'commondeps'
deps = ['src/app.js', 'src/other.js', 'src/files/*.js']

where various tasks can then depend on commondeps once to get that same list. Currently this pattern doesn't work but it should be able to properly carry the invalidation information through.