Add terminal autocompletion

[snappyapple632]

Issue Details

Autocompletion in the terminal is a simple quality of life feature implemented in many Linux programs that helps users avoid typos and save a bit of time. Currently, adguardvpn-cli does not have autocompletion.

Proposed solution

Add autocompletion for adguardvpn-cli commands (i.e. connect, disconnect, update).

Alternative solution

Implement a Unix-like options system for adguardvpn-cli (i.e. -c for "connect", -d for "disconnect", -u for "update").

[sfionov]

@snappyapple632 This is already done in #18 and included in 1.1 release

You may find the instruction in the install script output