add a `help` parameter for `genrule` targets that is returned in `plz query`

[bmustiata]

Hi,

First, thank you for the build system. I love it!

As I'm creating targets, especially via macros, it becomes difficult at some point to follow what target does what. It would make sense to me to be able to add help that will be associated with the target itself, where I can document in detail what's the purpose of that target.

So if there's a zip archive, I'd write what is supposed to zip.

So basically:

genrule(
  name="app#zip",
  cmd="zip ....",
  help="""\
  archives the application, and libraries for ....
""")