Add an option to customize ctags executable path

[marcsabat]

Summary

Add an option to customize ctags executable path.

Motivation

In Manjaro (and I think Arch linux) the ctags executable installed by default does not work properly.

Describe alternatives you've considered

I have opened a bug report at Arch Linux bug tracker:
https://bugs.archlinux.org/task/67651

However I think it might be a good thing anyway to have this option available for users to try different versions of ctags exec (even for developing purposes of this very same package).