add argument support

[skaldesh]

this adds argument support in a similar fashion as flags.
The following rules are implemented:

• arguments must be supplied in the exact same order as they have been registered
• arguments are now typed, just like flags (String, Int, Bool, ...)
• arguments can be optional and can then have a default value
• list arguments are possible, where the minimum and maximum number of elements can be specified
  Example: rm path [paths...] which has list argument path with min = 1
• it is not allowed to register an optional argument before a mandatory one
• it is not allowed to register more than one list argument, which must be at the end
• so only this order is possible: cmd arg1 arg2 [arg3] [arg4] [arg5...]

We make a small breaking change here and remove the old AllowArgs and refactor the Args on the context to use the new interface.

closes #3

[skaldesh]

An additional nice feature added here is, that if no usage message is defined, one is composed automatically from all the registered flags and args

[r0l1]

Awesome!