Py-pip

- forward arguments to run command correctly 90

- stop using unverified context 89

- Change installation instructions to use `pipx` PyPI name
- Add `ensurepath` command

- add version argument parsing back in (fixes regression)

- add version check, command check, fix printed version update installation instructions

- Replace `pipx BINARY` with `pipx run BINARY` to run a binary in an ephemeral environment. This is a breaking API
change so the major version has been incremented. (Issue 69)
- upgrade pip when upgrading packages (Issue 72)
- support --system-site-packages flag (Issue 64)