CreateTask derive macro should also create task options implementation

[joshradin]

As a continuation of #8 , the tasks whose CreateTask traited via the derive macro should be able to get values from the command line.

Proposed Method

A new annotation should be added to the CreateTask derive called option. Whether a value is optional, is a flag, and takes multiples values should be determined by the type of the field that's annotated by the option.

In addition, the option annotation should have 2 optional settings, name and parse.

• name accepts an alternative name for the flag. Otherwise the name of the field is used.
• parse accepts some function to try to parse a &str into type T returned in a Result<T, E>. If not specified, the FromStr::from_str is used if it exists. Otherwise a compile time error will be thrown

Example

The :task task is defined by the TasksReport struct, which could be implemented as so:

#[derive(CreateTask)]
pub struct TasksReport {
    #[option]
    all: bool,
    #[option(name = "group")]
    groups: Option<Vec<String>>
}