As using structopt without structopt_derive is quite meaningless, reexport
structopt_derive, symplifying the usage.
Inspired by the failure crate.
fix#45
* Add raw attributes
This makes it possible to call clap functions that don't take strings,
but any arbitrary value. Even functions that take more than one argument
can be called.
All attributes that are called `attribute_name_raw` are augmented to
`attribute_name(value)` without quoting `value`.
* Add tests for raw attributes and fix spelling
This changes the behavior of `from_attr_or_env` (used for clap
application settings) to favor the last occurrence of the requested
attribute. This is what repeatedly calling the same methods to set field
properties does implicitly.
