Allow flags to omit their default value in String()

This feature allows users to have a very clean and simple output for flags in --help output. I encountered this in a CLI tool I'm working on, where several string flags have no default value (ie: their actual default value is an empty string), and the resulting output for --help is a little ugly.

In order to accomplish this, I added a bool to some Flag structs, namely `OmitDefaultValue`. As a result of this, the default constructor for StringFlag, IntFlag, and Float64Flag. From what I can tell, few people in the Go community tend to use the default struct constructors without named parameters, but I wanted to point out that this is a breaking change.

With that said, I'm open to any suggestions to improve this PR. I had considered creating my own flags in my app with a custom `String()` func, but that seemed difficult for us to maintain, and I had hoped that other people might find this useful as well.

Example of the existing behavior:
```shell
>my-cool-app something --help
NAME:
   something - Does something really important, probably

USAGE:
   my-cool-app something [-b Blubber] [-c CLIs] [-d Detroit] [-e Everything]
               [-f Frumpy] [-g Grapes] [-h Hankering] [-i Indigo] [-j Jallopies]

OPTIONS:
   -b ''		Blubber and Bales
   -c ''		Cranky CLIs
   -d ''		Destruction of Detroit
   -e ''		Everything And More
   -f ''	    Frumpy Flags are Fruitful
   -g ''		Great Grapes
   -h ''		Headache Hankering
   -i ''		Indigo Iodine 
   -j ''		Jumping Jallopies
```

Desired behavior:
```shell
>my-cool-app something --help
NAME:
   something - Does something really important, probably

USAGE:
   my-cool-app something [-b Blubber] [-c CLIs] [-d Detroit] [-e Everything]
               [-f Frumpy] [-g Grapes] [-h Hankering] [-i Indigo] [-j Jallopies]

OPTIONS:
   -b 		Blubber and Bales
   -c 		Cranky CLIs
   -d 		Destruction of Detroit
   -e 		Everything And More
   -f 	    Frumpy Flags are Fruitful
   -g 		Great Grapes
   -h 		Headache Hankering
   -i 		Indigo Iodine 
   -j 		Jumping Jallopies
```