Add positional args support for fdl.Config.

[copybara-service]

Add positional args support for fdl.Config.

config.posargs returns the *args list and can be accessed directly.

To access positional args:

v = config.posargs  # the full list
v = config.posargs[-1]  # normal index
v = config.posargs[:3]  # slice index

v = config[-1]  # normal index
v = config[:]  # slice index

To modify positional args:

config.posargs = [1, 2]  # assign to a new list, 
config.posargs = config.posargs.append(3) # append one item
config.posargs = config.posargs + [3] # append one item

config[0] = 0
config[:] = [1, 2, 3]