Add headless support

[randshell]

I followed the style of the openvpn-install repository and used some of its code. Thanks to everyone that contributed there.

The other additions are:

• Allow customization of IP and NIC in headless mode, useful for servers with multiple IPs. openvpn-install repo will need this change too.

To consider:

• The headless addition of an user will block the script waiting for a free IP if the one inserted is already in use. I'd suggest to handle the IP management of the clients automatically as happens with OpenVPN and users that want a specific IP for a device can manually edit the configuration files. Other suggestions, for example exiting instead of asking again for an IP (it isn't very friendly when the script is run interactively), are welcome.

• The headless revocation of an user isn't friendly because we ask for a client number instead of a client name. openvpn-install repo will need this change too.

Changes aren't tested. PR is a draft while waiting for feedback on the above considerations @angristan .

Fix #21

EDIT: I've pushed some commits that would help with the above issues. Phrasing can be made more clear, but it's ready for review.

[deatharse]

@randomshell Any chance of a rebase to fix the current conflicts? @angristan Are you happy with the changes, is there anything else holding this up?

[Pionerd]

@angristan would love to see this included