feat(expressvpn): add plugin for ExpressVPN

This fix makes the autocompletion script also work on zsh without
changing how it works for Bash.

The expression "${COMP_WORDS[@]:2:$COMP_CWORD-2}" does not behave the
same on Bash and Zsh when the command has only zero or one arguments,
e.g. typing "expressvpn conne<TAB>" or "expressvpn <TAB>" would print
the error "autocomplete:8: substring expression: 1 < 2". This fixes it
by handling the case of the short command separately in a simpler way
and handling the rest of the cases the same way as before.

With this fix, zsh users just have to use the command "source
/usr/share/bash-completion/completions/expressvpn" to get the same
auto-completion as bash users.

File : /usr/share/bash-completion/completions/expressvpn

Diff to the original file (using the `diff` command) :

9a10,12
>         opts=$( ${COMP_WORDS[0]} "$cmd" "${COMP_WORDS[@]:2:$COMP_CWORD-2}" --generate-bash-completion )
>     else
>         opts=$( ${COMP_WORDS[0]} --generate-bash-completion )
11d13
<     opts=$( ${COMP_WORDS[0]} "$cmd" "${COMP_WORDS[@]:2:$COMP_CWORD-2}" --generate-bash-completion )

plugins/expressvpn/README.md

@@ -0,0 +1,14 @@
+# ExpressVPN
+
+This plugin provides completion support for [`ExpressVPN`](https://www.expressvpn.com/vpn-software/vpn-linux)
+command line interface on Linux.
+
+To use it, add expressvpn to the plugins array in your zshrc file.
+
+```zsh
+plugins=(... expressvpn)
+```
+
+## INSTALLATION NOTES
+
+Besides oh-my-zsh, `expressvpn` needs to be installed by following these steps: https://www.expressvpn.com/support/vpn-setup/app-for-linux/.