Verbose flag for 'help' command

[KyleBanks]

This resolves Issue #10.

To see how it looks, here's a modified copy of the existing Makefile:

# Run all tests.
test:
	@go test -cover ./...
.PHONY: test

# Install the program.
#
# For more details, this is a longer explanation of 
# what install does. I might even list some dependencies
# or trivia about the command.
install:
	@go install ./...
.PHONY: install

# Build release.
build:
	@gox -os="linux darwin windows openbsd" ./...
.PHONY: build

And the output it generates:

$ mmake help -v

  build:
    Build release.

  install:
    Install the program.
    
    For more details, this is a longer explanation of 
    what install does. I might even list some dependencies
    or trivia about the command.

  test:
    Run all tests.

And for comparison, the existing help commands:

$ mmake help

  build     Build release.
  install   Install the program.
  test      Run all tests.

$ mmake help install

  Install the program.
  
  For more details, this is a longer explanation of 
  what install does. I might even list some dependencies
  or trivia about the command.

[tj]

awesome! Thanks