Add map typed flags

[furdarius]

Issue #7
This PR add availability of map typed flags usage. Map keys can be string or kind of integer, and values can be any types available as flags by this library.

Example:

package main

import (
	"fmt"
	"log"
	"github.com/davecgh/go-spew/spew"
	"github.com/octago/sflags/gen/gflag"
	"flag"
)

type config struct {
	Field1 map[string]int64
	Field2 map[int64]int32
	Field3 map[int8]bool
	Field4 map[string]string
}

func main() {
	var cfg config

	err := gflag.ParseToDef(&cfg)
	if err != nil {
		log.Fatalf("err: %v", err)
	}

	flag.Parse()

	fmt.Printf("cfg: %s\n", spew.Sdump(cfg))

	/*
	INPUT:
	./flag -field1=test:123 -field2=8:2 -field2=3:5 -field3=5:true -field4=m:v
	*/

	/*
	OUTPUT:
	cfg: (main.config) {
	Field1: (map[string]int64) (len=1) {
		(string) (len=4) "test": (int64) 123
		},
	Field2: (map[int64]int32) (len=2) {
		(int64) 8: (int32) 2,
		(int64) 3: (int32) 5
		},
	Field3: (map[int8]bool) (len=1) {
		(int8) 5: (bool) true
		},
	Field4: (map[string]string) (len=1) {
		(string) (len=1) "m": (string) (len=1) "v"
		}
	}
	*/
}

TODO:

• Add tests for generated maps types
• Extend docs

[codecov-io]

Codecov Report

Merging #15 into master will increase coverage by 0.65%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #15      +/-   ##
==========================================
+ Coverage   99.29%   99.95%   +0.65%     
==========================================
  Files          10       10              
  Lines        1424     8042    +6618     
==========================================
+ Hits         1414     8038    +6624     
+ Misses          7        2       -5     
+ Partials        3        2       -1

Impacted Files	Coverage Δ
values_generated.go	100% <ø> (ø)	⬆️
parser.go	100% <100%> (+4.25%)	⬆️

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[m0sth8]

Could you add \nn here?

[furdarius]

Added.

[m0sth8]

Thank you!