Print task argument documentation to console

[yevhen]

Nake supports printing information about all available tasks to console via dotnet nake -T switch. This prints task name and description, which is parsed from method's xml documentation (either F# style or normal <summmary> tag). It would be cool to support more detailed task listing which also includes descriptions of task arguments.

The argument documentation may come from standard xml documentation of the task method. The user may invoke printing of detailed task description by passing --help switch after task name: dotnet build --help. If task has declared boolean argument of this name, the automatic printing is suppressed and task is called with help=true. This will allow users to override default behavior to print their own help.