Add support for registering/ retrieving env variables using goflags

[tarunKoyalwar]

Proposed Changes

• we use env variables as switches for many reasons some of them include

  • feasibility from implementation persepective
  • feature/switch does not need a flag (at least yet)
  • making library/SDK related switches available in downstream tools (ex: DEBUG=true for errorutil will print stacktrace)

• when env are registered/consumed from this implementation . all envs are available in help section of that tool (ex: nuclei -h envs) or something similar

• while goflags is primary developed for tools and not SDKs . this feature should be possible to be used in libraries without the need of making it a tool (see implementation) . this way all inherited envs are available in downstream tools

Implementation

• unlike flags this feature should be registred through global functions instead of methods from flagSet Type/Object etc (see ^ 3)

• below is example of using this in a library/tool

// something.go

var tlsVerify bool

// logic &
// code
// of 
// library


func init() {
   goflags.BoolEnv(&tlsVerify,"TLS_VERIFY","false","Enable/Disable TLS certificate verification")
}

since go executes in following behaviour global variables -> init() -> main() -------> there shouldn't be any trouble with logic

expected output is something similar to this

$ nuclei -h env

   TLS_VERIFY       (default: false)         Enable/Disable TLS Certificate Verification 
   DEBUG            (default: false)         Enable/Disable Printing StackTrace of errors