Auto add to path

Pros:
1. Removes an install step
2. Easier BC if we want to change where the CLI is installed (e.g. see this [installer](/pjcdawkins/platformsh-cli-installer))
3. Appears to work on Windows, so this unifies the configuration a bit

Cons:
1. My bash might be a bit ropey (?)
2. I moved the executable file (I don't think that's a BC break though as the proper installation method is via Composer)
3. Might not actually work on Windows