only rebuild those affected entries

[chinesedfan]

As #514 describes,

Add another build type, like mod. It may require neuron-builder to accept the changed file as an argument. So that it can reject unnecessary build. If users only modify a file in mod/, entries that do not depend on it will not be re-built.

This MR depends on another MR in cortex-neuron-builder project.

And I have to admit that function isMod is too rude. If reading from somewhere in cortex.json, say field mods, will be more flexible. But it may lead to a series of modifications.