v1.2.0: batch install, listing versions and help

Batch install support, listing versions and help

`terraform-install` now supports installing all terraform
versions referred in a tree in `.terraform-version` files.

It also gained capability to list locally cached and remotely
available terraform versions.

It also got a simple --help flag.

v1.1.0: auto install and direnv support

Add auto-install feature and direnv support with terraform-path

v1.0.0

Initial version