Allow optional flags by asking if its been set/present on the command line
Allow flags to omit their default value in String()

This allows you to have string flags with no value and to have a very
clean and pretty output for --help.
Merge pull request #57 from fraenkel/optional

Allow to detect optional flags
Merge pull request #58 from tjarratt/master

Allow flags to omit their default value in String()
Merge branch 'master' of https://github.com/codegangsta/cli