Feature: Add flag for install location (optional)

[ArronaxKP]

This change adds a new flag -i or --install that will allow you to set where the Terraform binary is downloaded/installed into. Useful for edge cases where you need to share commonly installed versions of Terraform.

This is in draft as it is built upon this PRs that should be merged first. To save headaches with merge conflicts this PR already includes that commit.

• #295 - Thanks @jukie for the PR.

Once the above PR has been merged ping me and I can update this branch with latest master.

I have created a similar PR for tgswitch here - https://github.com/warrensbox/tgswitch/pull/140