Flags refactoring + custom MTU

[cheina97]

This PR refactored flags using pflag packages.

The reason for this PR is to enable adding new flags to customize the wireguard parameters.
For example, the MTU has been added to the flags in this PR.

It also introduces the possibility to use the --help flag to information about the usage of wireguard-go:

Usage: ./wireguard-go [flags] <interface-name>
      --foreground   Remain in the foreground
      --mtu int      Set the MTU of the device (default 1420)
  -v, --version      Print the version number and exit
pflag: help requested 

[cheina97]

@zx2c4 how can I request a review?

[pronebird]

Can't you set mtu via wg wconfig?