feat: make where to install the completion files configurable

[wolfenrain]

Description

As a developer that does not like my system to be touched without consent from me, by any software, I love that we can disable the auto install. But it would be even better if there is an API that allows me to specify where to install. I could then hook this up on my own CLI to give my users more fine-grained control.

Requirements

• Being able to specify where to install the auto-completion files

[renancaraujo]

We have two levels in which this can be customized:

1. The package provides an API in which CLI maintainers can customize the install location (which is pretty much what you are suggesting, I guess).
2. The package provides an API in which the CLI users can customize the install location. For that, we can introduce a command that echoes the completion script and people can do this:

my_cli completion >> ~/.zshrc

[bartekpacia]

Looks like this issue can be closed, since (from the comment above):

• approach (1) is already possible
• approach (2) is tracked in issue feat: Add "print completion" command. #58