Fix init function

[jukie]

For environments without write access to /usr/local/bin, the existing function will fail because it attempts to remove the symlink if terraform already exists.
tfswitch should instead check for symlinks at the given binPath or default to user home directory.

[yermulnik]

This is not related to this PR, though I can see defaultBin definition in lib/install.go and can't make up my mind of why it is (re-)defined here (it is defined in main.go). Am I missing something?