Determine rule/args from command

[richfitz]

Replace things like

  growth_mortality_traits:
    depends: download/wright_2010.txt
    rule: process_wright_2010

with

 growth_mortality_traits:
    command: process_wright_2010("download/wright_2010.txt")

or more generally

target:
  depends: [foo, bar]
  rule: func

with

target:
  command: func(foo, bar)

Retain, at least for now the current depends: / rule: interface. An explicit depends will be needed for fake targets anyway.

The inverse problem to target_argument_name appears: how to pass in the name of the target into the function. Doing that manually is of course possible:

target.csv:
  command: f("target.csv")

But better would be to allow . (following dplyr) and/or more explicitly:

target.csv:
  command: f(target_name)

[richfitz]

This is basically done. The big outstanding issue is that we don't track if the command was given as

f("target.csv")

or

f(target.csv)

-- both are treated the same at the moment (eventually being converted to character). There is no environment in which things are looked up. That seems less than ideal, really.

To be stricter, could require that target names that are files are quoted and object names are unquoted, with special names . and target_name left unquoted. But that requires remembering which is which purely for the job of error checking. For the first version that comes up, probably just best to be well behaved and I'll pick this up when I do the next round of extra input checking.

[richfitz]

Leaving this open until the quoting issue is dealt with.