separate path for tfenv conf and terraform binaries

[waheedshahani]

Currently TFENV_CONFIG_DIR points to a path where you store binaries as well as tfenv conf. This creates a problem if you want to use tfenv to use multiple terraform versions at the same time. It would be nice to separate path location for tfenv configs and terraform binaries so that there is central location of binaries but we have possibility to use different location of tfenv configs specific to runtime executing terraform.

[jmnavarrol]

Maybe related to #251

[jmnavarrol]

See my last note on #251: it seems you can manage multiple versions of Terraform at the same time just by not setting TFENV_CONFIG_DIR.