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))

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. Not Windows compatible, although perhaps it could be with tweaks