Add an If command

[RomainGehrig]

If(a, b, c)

• First argument is a Python method that evaluates to True/False (by Python truthiness rules)
• Second argument is a command for the true branch
• Third argument is a command for the else branch, if given.

Design question: when should the first argument be evaluated ?

• At compose time: we only have access to config and args. Theoretically, only the arguments of the taken branch would be needed but the branch may change between invocations. Argparser would then complain of missing or unrecognized parameters.
• At run time: we have access to config, args and previous command output but we don't know which branch would be taken so we need arguments for both.

Alternative: StdinConverter can be (partially) used to replace the executed command at run time. Eg. StdinConverter(Cmd(sub=BranchA()), converter=lambda s: {} if not predicate(s) else {'sub': BranchB()}). However, it would not expose the arguments of BranchB and Cmd is a command that takes a subcommand as its argument.