Command line flag support 添加更多命令行选项支持

[ZeaKyX]

Description

Support more flag than -c, and maybe overriding config file? : )
添加配置文件里的选项到命令行参数中，比如可以运行speedtest -p 80指定网页端口号为80
默认优先级高于配置文件（？）

Why it should be implemented

1. Benefit who don‘t want to edit config file. 懒得写配置文件Orz
2. More easy way to set up some background service. 方便搭建后台

e.g

Maybe using pflag:

main.go

package main

import (
_ "time/tzdata"
"github.com/spf13/pflag"
"github.com/spf13/viper"
"github.com/librespeed/speedtest/config"
"github.com/librespeed/speedtest/database"
"github.com/librespeed/speedtest/results"
"github.com/librespeed/speedtest/web"
_ "github.com/breml/rootcerts"
log "github.com/sirupsen/logrus"
)

var (
optConfig = pflag.StringP("configfilepath", "c", "", "config file to be used, defaults to settings.toml in the same directory")
)

func init() {
pflag.StringP("listen_port", "p", "", "Listening Port")
pflag.StringP("proxyprotocol_port", "", "", "")
pflag.StringP("download_chunks", "d", "", "")
pflag.StringP("distance_unit", "", "", "")
pflag.StringP("enable_cors", "e", "", "")
pflag.StringP("statistics_password", "s", "", "")
pflag.StringP("redact_ip_addresses", "r", "", "")
pflag.StringP("database_type", "", "", "")
pflag.StringP("database_hostname", "", "", "")
pflag.StringP("database_name", "", "", "")
pflag.StringP("database_username", "", "", "")
viper.BindPFlags(pflag.CommandLine)
}

func main() {
pflag.Parse()
conf := config.Load(*optConfig)
viper.Unmarshal(&conf)
web.SetServerLocation(&conf)
results.Initialize(&conf)
database.SetDBInfo(&conf)
log.Fatal(web.ListenAndServe(&conf))
}

[maddie]

PR welcome :)