Allow arbitrary dependencies in command lines

[jeanlauliac]

Ex. I want to take babel as input in addition to the generating input files:

node node_modules/.bin/babel ......

babel needs to be a tracked input for the rule that runs this command line.

Plans is to have "dependencies" in addition to "order_only_dependencies" in the updfile, that will be treated just like inputs, their content being tracked, and can be inserted in the command line with a "dependency" variable. Each dep can be inserted separately in the command line. Deps can be sources or other rule outputs.