itkconfig: Add doc comments.
itkconfig: Fix golint issues. Decapitalize error strings.

The following golint issues have been fixed:
config.go:27:44: error strings should not be capitalized
config.go:33:44: error strings should not be capitalized
config.go:39:44: error strings should not be capitalized
config.go:45:44: error strings should not be capitalized
config.go:49:43: error strings should not be capitalized
config.go:60:21: error strings should not be capitalized
config.go:64:21: error strings should not be capitalized
config.go:98:26: error strings should not be capitalized
config.go:104:25: error strings should not be capitalized
config.go:128:22: error strings should not be capitalized and should not end with punctuation
config.go:134:22: error strings should not be capitalized
itkconfig: Make sure to close file descriptor after use.